Cary's Garage: Through Thick and Thin
I had a friend ask me recently about my experience with older air-cooled vehicles and what my feeling about the right oil weight to run was. Well, I have driven air-cooled vehicles for many years and many thousands of miles and have experimented with oil to see what had the best results.
I know for years it’s always been the mindset to run heavy oil because of the temperatures that these engines run at and the thought was that the best choice for lubrication was either a straight 30 or 20W50. Indeed, I was told to run one of these for a long time. Now, this isn’t including vehicles like the Porsche 911 that have a large capacity dry sump system, because they do spec out a heavier oil and they do well with that. But for vehicles like a Volkswagen, Corvair, Tatra, Fiat 500, Citroen 2CV, NSU, and any other air-cooled vehicle I have found that running a thinner oil has given better results for engine cooling.
The thing to remember is yes, the engine has a fan that blows air over the cylinder heads and cylinders to cool it down, but internally the oil absorbs heat and then goes through the oil cooler and that also is a big factor in keeping things cool. Where I live down in the Southwest during the summer months it can get pretty darn toasty and in general, I would worry about keeping my engines cool but never put too much thought into the type of oil I ran. I was of the mindset that this is what I was told, and it appears to work so I will just do it. However, years ago I was watching a video from a well-known car collector about his Tatra and how he achieves keeping the engine cool and that got me thinking that maybe there was something more to the whole thing than I had ever realized. So, I started to experiment with my own vehicles to see the effects for myself. Sure enough, it did make a pretty noticeable difference.
I installed oil temp and cylinder head temp gauges on various vehicles I was driving, so I could monitor how the engine was doing. I switched from my normal 20W50 and dropped to 15W40 oil and put down thousands of miles. Within that time, I saw a noticeable decrease in oil temps and even cylinder head temps while driving in different situations. Having a thinner oil that would allow the oil to move a bit quicker through the oil cooler allowed it to cool down more efficiently. So from there on out, I ran the thinner oils and even started to run a heavy-duty oil that had a higher zinc content to help protect some of the internals on the older engines.
One other thing to keep in mind: Switching to lighter oil alone won’t just magically lower the engine running temps. Having properly installed engine cooling tins and making sure there aren’t any air leaks is a very important thing to check as well.
Please send any questions to Carysgarage@gmail.com. Thanks!
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Thanks for posting this, Cary. Several years ago, we had a pretty in-depth presentation on oil, and viscosity versus thickness, temps, etc., at our local Corvair club meeting. The conclusions from the presentation (given by one of our members) pretty well matched your findings.