Rotella T straight 40 wt? shear stable?

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860
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SD - South Dakota
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?
 
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I always used 30w in my Volvo Penta IO that specified motor oil. In my Mercruiser I used the Merc/Quicksilver stern drive lube. 40w would be fine
 

meborder

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860
Location
SD - South Dakota
IIRC, the manual i bought recommended 40wt, which is why it caught my eye. it's confusing though, because the dipstick says to use 10w30, so currently it has Trop Artic syn blend 10w30. But it hasn't been run much either.... hasn't been in the water for 2 years, now frown
 
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Central Washington
Originally Posted By: meborder
IIRC, the manual i bought recommended 40wt, which is why it caught my eye. it's confusing though, because the dipstick says to use 10w30, so currently it has Trop Artic syn blend 10w30.
Thats probably becuase that is the same dipstick that motor got no matter if it was going to an S10, or being sold as a power package for a generator or boat..
 
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Saratoga, NY
meborder, looking at your original post, you seem to understand this issue well. Given that, I am wondering why you are considering using a straight 40? At least some of the reduction of weight in used marine oil is fuel dilution which will happen even if the oil doesn't shear a single point. Between a straight 40 and a 15W-40, I'd recommend the latter ... and suggest you might even be better off with a 5W-40. While the straight 40 oil is "interesting" as you say, I wouldn't bother with it, especially for occasional use in the Dakotas where the mornings can be a bit 'brisk.' If you were a commercial boat operator in the Florida Keys, I could see the interest.
 
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