Go to straight weight 30.
In theory multi viscosity oils act like the higher number when hot, but our little Marine
Diesels don't get that hot, so your running a less than the high number oil. Average modern automobile driven normally will have an oil temp in the low 200's, our little Diesels usually don't see much if any over 180. Most of us will never use our boats in temps that require a multi vis oil.
If you want to run a multi vis, go for a 15W-40.
My 30 yr old 4JHE using Shell Rotella T6 synthetic 5W-40, burned about a quart every 100 hours and didn't smoke, but left a greasy stain on the stern when I had a following sea. Now Rotella T6 sun is an excellent oil, just I think maybe a little thin for my old Diesel.
I am now using Rotella straight 30W at about half the oil consumption and no greasy stain.
Now I am a strong proponent for Syn oil, I am not sayin Syn is bad, it is not, it is I believe superior, but my old motor
, the 5W-40 is just too thin, and or possibly doesn't burn at the temps I have?
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