Wow there seems to be more to this oil question than I first imagined!
I've always assumed oil to be much of a muchness!
I found this one on a 500 website and he seems very confident of what he says which backs-up alot of the Mineral oil guys out there:
For those who like reading and want another point of view, below is reproduced an edited version of the article I referred to earlier which is aimed at the VW aircooled owner but the same principles apply to a 500.....
This article and the information contained in it, is truly independant of any fuel company or product manufacturer. There is no bias, blame or solicited promotion. It is written by a professional fuels chemist whose hobby for over 12 years has been restoring aircooled VWs.
Engine oil
There are many myths abounding about oil - which is no surprise as oil is doubly important in an aircooled engine. Why is it important? Two reasons - an aircooled engine runs hotter than its water cooled brethren. and also because oil has a secondary duty of assisting in extracting heat from the engine.
Two such myths are explained:
Myth One - "use diesel oil"
Many many many (etc) years ago, there was a diesel oil made by Shell which was a wonderful substitute for the correct oil for the Beetle. This was about the time that multigrades were in their infancy on the forecourt. Many small non VW garages swore by this certain diesel engine oil - and so they should have - it did the right job. HOWEVER this oil can no longer be bought and there is no substitute - and just like chinese whispers, the detail gets lost as rumours spread from generation to generation.
DO NOT USE DIESEL ENGINE OIL UNLESS IT ALSO HAS AN API RATING OF AT LEAST "SH"
Myth Two - "don't use multigrade"
This myth stems from the early days of multigrades when the viscosity index improvers of different brands often clashed, with bad results. This meant that you couldn't top your oil up with another brand.
With modern day oils this is no longer a problem.
Synthetic oil
Because of the formulation process (and the increased advertising that these oils have), synthetic oils are also more expensive than mineral oils - but are they right for a street VW? The simple and current answer is NO. Although one or two oil companies are working on synthetic oils with improved thermal absorption, most synthetic oils do not absorb heat well. This may be good news for a water cooled Le Mans racer, but this is extremely bad news for an air cooled car such as a VW or Porsche where oil has a secondary role as a coolant.
Summary
For a street aircooled VW, regardless of age or whether itis stock or gets used on the strip at weekends, use an SAE 10W40 multigrade with a minimum of SH rating. Never use diesel engine oil (unless it has a minimum API SH grade too).
