
Oil lubricates, cools and carries dirt away, and it can fall short at all three long before anything shows on the dipstick. Diesel in the oil thins it, heat makes it age faster, blow-by gases bring in soot and acids.
If all you want to know is which oil belongs in your engine, go straight to Which oil?. If you would rather clear up the common misconceptions first, start with the oil myths fact check.