The way I look at it, if there was 1 most important thing in terms of your WHOLE car's maintenance, I would think its engine oil. Secondly, for me, I change my cars' engine oil at 5k kms fixed. Which equates to about 6-7 months for my Turbo and about 8-9 months for CL9.
Now there's no way to really test the quality of oils we are using, but due to above reasons I try to go for high quality oils. Reason I choose Liquimoly is that its supply is common, even though there are better oils available. I don't want to be switching oil brands often. I've analyzed the technical data sheets of the Liquimoly oils I'm using and since it comes direct from Germany, it personally gives me more assurance that the oil is exactly as per specifications mentioned in the technical data sheets.