If you want to know more about engine oil, maybe more than you wanted to visit this site. It was well written and as you can tell from the author's verbage and references he is from Europe. This was sent to me from a site visitor and it took me a while to read it, I get a lot of mail and referrals, but this is informative and useful.
His explanations are a little involved but if you take the time to complete each section he clears up the message. One word of caution is that read the whole article before you stop half way and try some of his ideas, you may reconsider