Oil Viscosity

Why is 20w-50 the std weight oil in our air cooled cars?

Why not 10w-30?

An interesting question. The Owner's Manual for my '72 specifies SAE 30 for most warm temps and an SAE 20 for winter (and SAE 10W for very cold temps). My, how far we have come in terms of engine oil technology!

Many now use SAE 20W-50, based on the logic that the "hot temp" 50 weight will offer better protection for our air-cooled cars. I personally believe that this is overkill and use an SAE 15W-40 heavy duty motor oil (Delo 400) for the added wear/antioxidant protection and the "cold temp" SAE 15W works fine in California at start-up.

For reference, not that long ago, the factory oil was a synthetic SAE 5W-40. The SAE 10W-30 oil will most likely work fine. If you race and your engine runs hot, a higher viscosity oil might be a good idea.

That makes sense., and I suppose I should have been more specific in my question since its not the factory recommendation of 20w50. The last two shops that changed my oil were both high performance Porsche shops. I never bothered to ask why they use that weight oil. I'm in the Northeast and the cold starts are getting pretty frequent.:(

If you run your engine hard and hot all the time (i.e. race), I could go with the anecdotal position of using a 20W-50. You probably don't race in the winter so the 20W starting vis would not present a challenge. If you only do cruises and change your oil regularly, 10W-30 will work just fine.

The factory did, in fact, recommend 20W50 for our older cars. They revised their recommendations about 10 years ago (I remember Bruce Anderson posting them in Excellence). If I recall correctly, 20W50 for Spring through Fall, and 10W30 for Winter (or something close). They also didn't recommend synthetics if your engine had older seals due to the possibility of increasing leaks.

Speaking of our leak-prone Porsches. My '71 leaked more running 15w40 than it does with 20w50.

Unless you live somewhere very cold, you should have no concerns about starting your car with 20 weight oil.

Oil is like religion for most people (a touchy subject). If you've got good oil pressure with 10W30 and she ain't leakin', cool.

My 1.5 cents.

John