Prescribing a non-synthetic 15W40 is a bit strange. I have been reading a bit about engine oil on Wikipedia, and thinking.
A 15W40 non-synthetic seems to be a basic oil of 15W quality, meaning viscosity 15 when cold. A single
grade would become too thin when hot; therefore dopes have been added which increase the viscosity to 40 when hot.
Thing is that these dopes are rapidly destroyed by the action of the gears, effectively reducing the oil from 15W40 to 15W30 to 15W20 etcetera.
This effect is a good reason for taking a synthetic oil, which according to Wikipedia has a "flatter" curve of viscosity vs temperature.
BUT. I guess the oil in the saildrive
does not become hot at all. Of course the gears are disspipating some horsepowers, but the cooling
of the saildrives could hardly be bettered, unless you were motoring in one of the ponds in Yellowstone or Iceland
. But in those cases where you are actually sailing in seawater below 30 centigrade I guess the oil would be used in the 15W range.
So if the reason is not the multigrade, what is the reason for prescribing this oil? Maybe some anti-wear properties for these same plates?
It would be nice if Volvo Penta
could shed some light.
And Lori, yes I know of a boat which had worn plates. After 2 years/20,000nm (skippered charter) the friction plates (for switching FW <-> REV) were worn out and replaced under warranty. Maybe there is a relationship.