VW Baywindow Bus - Engine Oil Selection
by Richard Atwell
Which motor oil should you use in your VW bus? Let's see if we can come to some sensible conclusions to help maximize engine longevity.
By the end of these articles, I hope to have assisted you by giving you the knowledge to select the best oil (or oils) for your air-cooled VW engine that:
The choice of brand is up to you but after satisfying the list above, the selection narrows to a handful of marketplace choices...
Back in the days when John Muir was still fixing VWs and oil came in cans, multi-grade oils were still somewhat experimental.
Eventually the oil companies formulated them into the reliable products we use today and by the mid-70's, VW had published an engine oil viscosity specification chart that superseded all prior advice on oil selection. They recommended several multi-grade oils along with traditional single-grade oils so what's the difference?
The idea behind multi-grade oil is simple. Take 10W-30 as an example: behave like a SAE 10W oil at 32F (in order to more easily lubricate the engine during startup) and behave like a SAE 30 oil at 212F for protection at normal running temperatures. This is something that a single-grade oil cannot do as well.
Here's a chart that shows the how's the viscosities of 10W, 30 and 10W-30 change with temperature:
This is accomplished by using a base oil (e.g. 10 weight) and adding viscosity modifiers which "expand" and thicken as the oil heats up.
This video nicely explains how the naming scheme works:
VW makes recommendations about grade (weight, viscosity, etc) in your owner's manual using a generic industry provided temperature vs. viscosity graph.
However, there is often a debate whether or not this information out of date and while manual touches upon driving habits it's not clear whether you should use a single-grade or multi-grade engine oil.
Here's what the 78 owner's manual has to say (I've drawn a red line at the top of the 20W-50 heat range where the 1978 multi-grade oils were apparently no longer effective):
According to the chart, you shouldn't be driving around in 75F temperatures (24C) or hotter weather unless you have filled the sump with SAE 40. Some classic VW owners have taken this as gospel and still put SAE 30 or SAE 40 into their engines fearing they may overheat if they use 20W-50 (as the chart suggests). It's a valid concern but what's the reason for the recommendations in the chart?
In the 70's the 20W-50 formula came with an SE rating from the American Petroleum Institute (API). When used under these conditions, it couldn't sustain the heat and would break down too quickly in the hot running air-cooled engine (probably a lot of other engines too). This is a definition of the SE grade from API:
SE: This category was recommended for certain 1971 vehicles as well as most 1972 vehicles. This classification offered more protection than the SD group of lubricants and was suitable for severe-duty applications. This classification is used in place of SD oils, but it is now obsolete. - source Valvoline
Going by the API classifications alone, oil formulas didn't improve for 16 years! until the SF rated oils appeared:
SF: Recommended with 1988 and older passenger vehicles. This oil has superior anti-wear properties and enhanced oxidation stability over SE lubricants. - source Valvoline
Oxidation of oil is accelerated at elevated temperatures: the cylinder walls get very hot and the oil there tends to vaporize. Oil is also sucked into the combustion chamber past the piston rings due to the vacuum generated by the piston travel and as a result the engine "burns oil".
It's easy to see why some people don't trust multi-grade oils: it's clearly marked on the chart but is trying to match 70's oils to 70's engine technology the best approach?
Here's some evidence that shows VW's oil viscosity chart from the 70's is truly out dated: VW continued to update the chart across the product line.
Now, compare both charts to the original in the 1978 owner's manual and you can see as the oil formulas improved so did their resistance to high temperature (at least according to VW). To summarize the improvements by comparing 20W-50:
Now what if VW had continued making air-cooled engines in Germany into the mid-late 90's? The oil chart may have looked like this (from a 95 Audi owner's manual) and pushed multi-grade 15W-50 for example to plus 40ºC:
The original VW beetle was still in production in Mexico until 2004 so what oil did VW recommend for that engine and climate? Here's an excerpt from the German version of the 1600i (ACD engine) technical manual (1994):
Translation: "Only use SAE 15W-40 SF oil approved by VW of Mexico". Unlike the beetle manuals of old, there is no climate chart but these guidelines are specific to Mexico.
It's interesting to note that VW was still recommending a SF oil (1980-1988) in Mexico 1994 but their recommendation closely matches the water-cooled engine oil advice from VW/Audi.
As of 2020, we're up to API SP now and so I think it's safe to conclude that modern multi-grade oils can be substituted for SAE 30 and SAE 40 especially as the single-grade oils were eliminated from the charts over the years. The updated charts also address the concern that using a multi-grade at its upper limit is going to wear it out because that limit has increased with subsequent API designations.
Not to say that a single-grade oil doesn't work; it's just that single-grade oils have a narrow temperature range and are not the only safe and viable option as is often stated by some in the classic car community.
Still not convinced? Updated recommendations were published in the official service manual (Bentley) specifically stating that:
For a long time, multi-grades with overlapping viscosities have been available which cover a greater range of operating temperatures. They work to better protect the engine throughout the temperature swings that the engine experiences. Also consider that 5W, 10W and 20W-20 are no long sold on the shelf because they have been surpassed by newer and better engineered multi-grade oils.
This table contains kinematic viscosity data from Valvoline SP conventional oils and is sorted by the viscosity at 100ºC.
|SAE Grade||5W-20||5W-30||SAE 30||10W-30||SAE 40||10W-40||20W-50||SAE 50|
|Viscosity @ 40ºC/104ºF (cSt)||50||70||87||71||125||111||179||231|
|Viscosity @ 100ºC/212ºF (cSt)||8.5||11.3||10.3||10.5||13.5||15.7||19||20.0|
|Pour Point (ºC)||-36||-36||-18||-33||-27||-33||-15||-9|
The viscosity of each oil is tested at two elevated temperatures: 40ºC (104ºF) and 100ºC (212ºF). The unit for viscosity is Centistokes (cSt): the higher the number the thicker an oil will be at that temperature and the less easily it will flow.
Note: 100ºC is sometimes cited as the temperature that any motor oil will achieve its maximum lubricity (unconfirmed). Other times it's cited as a typical operating temperature for the average engine.
Comparing the flow characteristics of two grades of oil (SAE 40 vs. 20W-50) to see how they differ, the product data sheets tell us that Valvoline SAE 40 is actually a thinner oil than Valvoline 20W-50 at ALL test temperatures. This is contrary to VW's oil chart.
After looking up the technical data sheets for modern oils and discovering the more recent charts from VW, I think at this point we can conclude that the chart in the owner's manual is completely out of date and stuck in the SE era.
As charted, there are drastic changes in viscosity between SAE 30, SAE 40 (and SAE 50) which at one time were the only summertime options. In very hot climates like Texas/Florida, SAE 50 would sometimes go in the crankcase according to some VW owners who lived in such places even though it's twice as viscous as SAE 30. In mild climates, SAE 30 might stay in the crankcase all year long.
Judging by the data alone, 10W-30 is a better choice than SAE 30 because it flows better at lower temperatures and has similar viscosities at 40ºC and 100ºC to SAE 30. At operating temperature both SAE 30 and SAE 40 are slightly thinner than their multi-grade counterparts at least initially.
I put together an updated chart for 2020 using the latest data for several grades of oil to show the relationship between different viscosities.
You can see that at the low temperature spec (40ºC) [blue bar] there is a huge spread in viscosity which are roughly divided into three groups.
While viscosity at ambient temperature and maximum pour points vary by grade quite a bit, the range of operating temperatures is much smaller which makes sense given that the heat created by combustion and friction is much greater than variations in ambient temperature.
As the oils warm up (70ºF) viscosities start to even out in the middle of the pack [green bar]; 80ºC [yellow bar] and by 100ºC [red bar] there is only a 2:1 difference in viscosity between the thickest vs. thinnest oils.
NOTE: the [blue/red] column values were taken directly from the TDS for each brand/grade. The [green/yellow] bars in the middle were generated using a calculator for kinematic velocity in order to compare grades at the temperature that VW specifies for oil pressure tests: 70ºC (158ºF) or 80ºC (176ºF). The graph is pseudo-sorted by these warmup values.
It's also interesting to note how similar "neighbouring" grades are with respect to each other. There's clearly 4 cool/cold weather grades to the right, 3 extreme weather or "race grade" oils to the left and in the middle, the warm weather grades.
Chart Highlights (these brands/grades only):
So, which one are you going to choose from the spectrum? You need a low pour point yet a thick enough oil to get the oil pressure and temperature at highway speed in the "green zone".
As SAE 30 seems "middle of the road" between SAE 40 and SAE 50, why not use it all the time? According to VW at the time of manufacture, the temperature range of SAE 30 was limited to above freezing and why 20W-20 (an early and obsolete multi-grade of oil) was introduced.
Both charts are clearly from the transition period from single-grade to multi-grade engine oils.
Take heed of the warnings on those pages about high-speed driving using the single-grade winter oils (SAE 5W and 10W). Fortunately these grades are no longer on the shelf.
Before the multi-grades appeared you HAD to select the best oil for the season. If you left SAE 40 in the crankcase as winter came, the starter would struggle to crank over the engine and you'd risk increased wear. If you switched to a thinner 5W grade in winter and left it in, the engine oil would thin out too much and turn on the low oil pressure warning light.
The lesson here is that highway travel is UNWISE unless the grade of oil falls into the acceptable ambient temperature range.
Think of your driving habits on a vacation: a lot of starts and stops for sightseeing, gasoline, snacks, potty breaks, etc. A 5W- oil excels under these conditions but combined with hot weather you run out of capability with a 5W-40 and need a thicker oil like 5W-50.
Whatever grade of oil you are using, engine wear is greatest during startup while the oil pathways are still pressurizing. It has also been said that starting your engine is the equivalent of driving 500 miles in terms of wear. Only running out of oil or overheating the engine is more damaging.
We assume oil pressure rises instantaneous because we can't see what's going on. When the oil pressure light goes out it gives the impression that optimal oil pressure has been reached but the time it takes is dependent on the temperature and the grade of oil.
Watch this video (at 5m30s) to see how many rotations it takes for the whole camshaft to get lubricated using 5W-30 with an inline 4-cylinder engine. Keep watching to see what happens to 10W-40 in extreme cold: 5s to get pressed out of cam tower #1 and 1s later for each successive cam tower.
Air-cooled VW's don't have cam towers that we can look at because the camshaft is hidden within the engine but the principle is the same.
Although these tests were done in extreme cold weather test conditions it serves to demonstrate how much difference there is between 5W- and 10W- grades and why choosing the right oil matters.
Are we getting closer to making a decision yet?
As much as we think we need thicker oils for the hot running air-cooled VW engine, as a best practice you should switch away from thicker grades of oil like 15W-40, 15W-50 and 20W-50 as the weather turns cold for the sake of engine longevity.
The ability of multi-grades (especially synthetics) to flow more easily at low temperatures than single-grades like SAE 30, permit oil to reach the bearings during startup more quickly. This lets the oil work as designed: as a lubricant relying less on anti-wear additives.
The engine always needs an oil that can flow easily at or below freezing if you live in such a climate but I think it's equally important to consider how well any oil flows at the ambient startup temperature, not just during the winter months.
What happens in between the lowest pour point and 100ºC varies by brand and by grade. While the viscosity charts like the one I created are useful indicators of viscosity under test in the lab, some brands/grades simply flow more easily than others at startup temperatures and its illustrative to compare oils against each other.
So, inspired by YouTube and to demonstrate the variability in viscosity and the slippery numbering scheme for oil grades, here's a simple test I performed with several brand/grades of oil from my garage:
Tip: I used my fridge freezer to cool the oil below freezing and I used a hair dryer to get the oil temperature up to 30-40ºC. Next time I want to try out a new oil, I'll swap that one in for the unlabelled vial and run the test again.
These results are in line with the viscosity chart but just looking at the 5W- on the bottle tricks you into thinking 5W-50 flows better than you expect. This is why looking up the product data sheet is so important before pouring the oil into the crankcase.
The expectation that single-grade oils are not designed for cold weather is also confirmed. SAE 40 would be even thicker and SAE 50 would finish last keeping in mind that VW never recommended SAE 50.
Also seen in the video: as the oil temperature increases, the differences in viscosity start to even out and you just need to go by the oil grade charts to determine if you would pick a multi-grade that ends with -30, -40 or -50 based on the seasonal outside temperature. Overall there is a 2:1 difference in viscosity across the range.
Multi-grades offer the best of cold and hot temperature engine protection just as designed but sticking with the same multi-grade year round will be dependent on limited temperature swings where you live.
This is one reason why there's no one perfect universal year round oil unless you happen to live near the equator where the seasonal temperature is year round. That's the "sometimes" from above. Many grades of oils are simply a compromise which serve to simplify servicing.
Given that the SH chart tops out at 15ºC for 5W-40 and 10W-40, using a 5W-50, 10W-50, 15W-50 or 20W-50 multi-grade oil in summer above those temps is good practice.
As a rule of thumb where I live (West Coast, Canada), using my viscosity chart, seeing how several oils flow, I believe 5W-50 is a versatile year-round grade for an air-cooled VW bus engine. Anything ending in -30 is just too thin where I live.
The alternate approach is to use 5W-40 in winter and 5W-50 in summer for highway driving as better choices. Given the seasonal oil change requirements, this is the best approach.
As a practical matter, I can easily find 5W-50 with the correct amount of additives (see article #3 for ZDDP info) at the local suppliers but with 5W-40 I have to mail order it or special order it locally.
Color has nothing to do with viscosity. It has more to do with the makeup of the additive package.
Royal Purple motor looks purple because of added dye; Redline is dyed red. Some vintage conventional oils used to be green in color and some companies dye their modern oils green to sell to that "good ole crude oil from location X" minded segment of the public. Others like Liqui-Moly are dyed fluorescent green so they can be seen under UV light to source oil leaks. 2-stoke oils are often dyed blue.
Here's good video on the components that make up an engine oil that depict the coloring:
I've written another article devoted to synthetic oils but it's worth mentioning here that Porsche produced a set of guidelines for their 968 that demonstrates their approval of thinner grades of synthetic oil, notably with lower (W)inter numbers than conventional (mineral) oils. For example:
What Porsche is suggesting by recommendation is that you can and should use a synthetic oil with the lowest pour point that still meets the operating temperature stability that's required for your climate.
An old school argument claims that the long hydrocarbon chains that make up oil get chopped up by contact with the engine parts (i.e. the oil breaks down). The argument then goes on to state that a multi-grade oil like 10W-40 eventually becomes a 30 at running temperature, then 20, etc. until it becomes the "base oil" used to create the multi-grade.
While this is true but it's also true of single-grade oils and based on experience of the SE oils from the 70's people decided to "stick with what works": the single-grade oils that have always been available.
The argument then goes on to state that the single-grade oils are more resistance to this kind of breakdown but I have yet to read anywhere that this has been conclusively proven as significant. It could be the case that the flash points of single-grade oil has always been high and the 70's multi-grades weren't as well engineered as they are today and didn't last as long but we can only speculate about this.
There's also been a belief that the spread between the numbers should not exceed 30 points (e.g. 10W-40 or 20W-50). Oils now available with 45-50 point spreads seen in a 5W-50 or 10W-60 grades tend to be high-quality synthetic based formulas which were formulated from multi-grade synthetic base oils and resist this kind of breakdown.
For example, Joh Muir's "Rap on Oil" (How to Keep Your Volkswagen Alive - Page 94) recommended you only use SAE 30 (Castrol specifically) because "the multi-grade oils' ability to lubricate is not as good as the mono-grade oil's ability at the high temperatures at which and air-cooled engine runs". Again, this was in the SE era and is now out of date advice.
It's a given that engine oil thins out as it wears and loses viscosity but is this significant in a multi-grade oil today? Viscosity breakdown is one of the reasons why we change oil at regular service intervals.
To truly find out which type of oil is best, we'd have to send off 4 samples to an oil testing facility:
In addition, we should take oil pressure and oil temperature measurements (at the start and end of each drain interval) to add to the data set and perhaps add an extended drain interval oil change to better demonstrate any differences.
The total cost for the tests will be about $120 ($30/sample) and it will tell you which oil lasts the longest in your 40 year old VW engine but you'll have to get it done before the seasons change.
While that would be conclusive, I haven't done such a single vs. multi-grade "oil analysis bake-off" as I suggest above. What I have done is send off multi-grade oil for sampling a few times for my own piece of mind and the reports have always come back indicating no significant breakdown in viscosity.
As a result, I'm not worried about leaving a single-grade oil on the shelf especially given it's poor ability to flow at colder temperatures. I firmly believe that the lower pour point benefits of using the multi-grades outweigh the "perceived durability" of the single-grades especially if you change your oil often. If you are concerned about durability, test your oil at the end of the oil change interval.
Compare this vintage can of Quaker State oil from the 1950's for an example of oil volatility when subjected to heat (evaporative loss). During this period, which region of the USA your crude oil came from actually mattered. These days most oils are made from equally high quality crude oils with the exception of some high end Pennzoil oils (Ultra) which are made from natural gas!
If you find an old bottle of oil, this table will help you date it. You can also see the intended goal for each additive package that meets the API certification.
In addition, I've added the ZDDP levels which have changed over time (where I've been able to source the P/Zn limits).
For more detail on the timeline, see Oil for Classic British Sports Cars.
|Service Category||Years||Description||Phosphorus (ppm)||Zinc (ppm)|
|SA||up to 1929||This is a plain mineral oil that doesn't contain additives common in today's high-tech lubricants. This oil was primarily used in the 1920s.|
|SB||1930-1963||Lubes that contain anti-wear and oxidation inhibitors as well as corrosion inhibitors. This oil was primarily in use prior to 1964, was created for vehicles that saw moderate conditions. ZDDP patented by Lubri-Zol circa 1941.||~300 (1942)
|SC||1964-1967||This classification was originally recommended for use in 1964-67 vehicles. It contains additives that control rust, wear, corrosion and engine deposits.||800 (since 1955)|
|SD||1968-1970||SD lubes were recommended for use in 1968-70 vehicles as well as certain post-1970 passenger cars. This oil contains the same additive packages as the SC class and can be used in place of it.||800 (since 1955)|
|SE||1971-1979||This category was recommended for certain 1971 vehicles as well as most 1972 vehicles. This classification offered more protection than the SD group of lubricants and was suitable for severeduty applications. This classification is used in place of SD oils.||~1000|
|SF||1980-1988||Recommended with 1988 and older passenger vehicles. This oil has superior anti-wear properties and enhanced oxidation stability over SE lubricants.||~1200-1500|
|SG||1989-1993||The SG rating was introduced in 1989 and combined the performance properties of the commercial rating CC (lubricants designed for use in supercharged/turbocharged diesel applications in moderate to severe service). Its designated use is for 1993 and older engines.||~1000-1200|
|SH||1994-1996||SH was designed for 1996 and older engines. This is the ZDDP level that Mobil 1 has targeted for flat tappets.||1200||1300|
|SJ||1997-2001||This is the grade where the ZDDP levels started to drop in many formulations and first gave rise to camshaft wear issues.||1000||1100|
|SL||2002-2004||SL oils are manufactured for better high temperature deposit control and lower oil consumption. Some SL oils also qualify as "Energy Conserving".||1000||1100|
|SM||2005-2010||Improved oxidation resistance, deposit and wear protection, and low temperature performance. This is the grade where the ZDDP dropped to levels that were too low to support older engines with flat tappets or high lift camshafts.||800||870|
|SN||2011-2010||Improved high temperature deposit protection for pistons, more stringent sludge control and seal compatibility.||800||870|
|SN Plus||2018-2020||Supplements the API SN standard with the addition of low speed pre-ignition (LSPI) protection.||800||870|
|SP||2021-up||API's latest standard with enhanced protections against LSPI and timing chain wear.|
Note: the P/Zn levels are maximums for 10W-30 or thinner oils to meet the API specifications only. Regardless you still need to obtain the exact levels from the manufacturer or by virgin oil analysis.
Sources: Google and API references below.
03/16/07 - Created
09/08/11 - Fixed broken photos, added translate button, updated footer
07/15/19 - Google update: new adsense code, removed defunt translate button
10/15/20 - Updated links and the API rating to SP, added my own preferences, warnings about oil additives, and VW oil service documentation
11/08/20 - Added some common sense ZDDP advice and GM EOS photos
12/13/20 - Added some warnings and videos on oil additives
01/02/21 - Added pour test video and split article into three separate articles
01/04/21 - Added 968 SJ-chart
01/12/21 - Added Bentley advice from Lubrication and Maintenance Pg. 4