I saw some Rotella T straight weight SAE 40 today at TSC and got to wondering if that would be a good choice for my older Stern drive. it is a 1988 chevy based 4.3, OMC Cobra stern drive. Carb'd if that makes a difference. the maintenance manual i bought says not to use multi-grade oils, presumably because multigrades of the day were more likely to shear under sustained high rpm use (not that it revs all that high, 4,600 rpm max). i know lots of people are using 15w40, and that would be fine for me too. but i saw the SAE 40 and it looks like it might be worth considering. as i understand it, the 25w40 they sell at the marina is basically a straight 40wt with a "w" rating, that is to say that most 40wts will pass a 25w rating as well. thoughts?