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Hello, I would like to ask you what type of engine oil do you use? I am currently using 15w-50 in my M10 engine.
Do you think that it is a good choice or should I consider a different type for my next oil change?
Also synthetic, semi synthetic or mineral do you think is the best for our cars?
In case it's important as an information, I live in a warm climate and I daily use my e21.

My Haynes Manual from 1981 specifies 20/50, actually back in 1982 there wasn't that much choice and that was almost all you could buy, everything ran on it and there was no such thing as synthetic. Also the tolerances in engine manufacture back then were not as close as they are now so I'd probably go with the 15/40, mineral or synthetic. Personally I think regular oil changes are as important, if not more important than the type of oil you use.
A friend of mine ran an old air cooled Beetle, he loved it and thought putting Mobil1 in the sump was a good thing to do. It leaked from everywhere possible, past every gasket and oil seal, what it didn't leak it burnt. He went back to mineral 15/40 and all was well.

There is a Dutch topic about it: http://bmwe21.net/forum/viewtopic.php?f=60&t=24461

I have now the Mobil Peak Life 5W50 in my M10 and after 2500km the oil is still clear. In your climate you could think about a 15 or even a 20 when cold. For the older engines a viscosity of 50 when warm is never wrong!

I thank you both for your answers. Based on those I am now between 15-40 and 15-50. I have found 3 brands which I believe are good for me. Liqui Moly, Motul and Valvoline. Which one would you suggest me; Personally I've use Valvoline before and it was ok. But I want to know if you think that one of the others is better.

I feel for our oldies Motul 300V Competition 15W50 currently has the best papers