Join Date: Apr 2012
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Re: Confused about which oil weight to use
I recently switched to Shell Rotella 15W-40 in my Yanmar 2GM and it is now easier to start when cold. The recommended straight 30-weight oil seemed too thick for winter and loaded up the starter more (as you would expect). This engine is also raw-water cooled and never gets very hot. To flush out the old 30-weight completely I did an oil change after about three hours on the Rotella and now do it twice a year, regardless of hours run (my engine does big hours by pleasure boat standards).
It is said that any damage to an engine happens in the few seconds after starting, before the oil gets to all the bearings etc, so lightweight has to be a good thing. Even though the manufacturers always want you to use their own expensive 'home' brand oil, I have run a fairly cheap locally-produced 15W-60 semi-synthetic diesel-engine oil in air-cooled dry-clutch motorcycles for many years with no issues.
With oil, you will get dozens of different opinions, sometimes clouded by a commercial interest. Mine is that any oil these days has to be reasonably good to make it to market and I would place more importance on twice-yearly oil changes than any particular brand. Shell Rotella is as good as any, but if I couldn't get it I would buy whatever grade and brand suited the engine. My other belief is that there is nothing wrong with mineral oil in a marine application, and in fact, full synthetic is probably a waste of money (ducks for cover.....).
If it is recommended to use straight 30 in your gearbox, stay with that. Unless it uses something else, like hydraulic fluid, of course!