Some time back I came across a  query on whether synthetic oil could be safely used in marine diesels. My response:

Most manufacturers permit the use of synthetics, but don’t allow longer oil change intervals when employing them.  The question I’ve always had is whether the gain is worth the cost.  Some of the advantages of synthetics that spring to mind are 1) better performance at temperature extremes, and 2) slightly lower engine internal friction.  In the past, when racing cars, we used synthetic engine oil at times on the premise that synthetics would provide adequate lubrication for very high load applications using lower viscosity oil.  We were after the slight increase in usable horsepower provided by the small decrease in internal engine resistance obtained using a thinner oil.  I believe this is likely measurable, but I don’t know if it’s really significant.  I somewhat suspect that it’s close to an irrelevant gain but, when racing, we would take every trick we could get even if the gains were slight.  I feel less inclined with recreational marine diesels and there is no way I would recommend using a lower viscosity oil than specified by the manufacturer, whether synthetic or not.

On the temperature extreme front, we felt that synthetics would allow us to operate the race engine longer before catastrophic failure when an engine was failing with low oil pressure or overheating.  We might be able to get a lap or two more before it completely stopped operating.  Overheating a diesel is close to the worse thing you can do, so the ability to operate somewhat longer under these conditions is not something I’m willing to pay all that much for.  However, if you live in the arctic, the ability to start easily and get better lubrication faster on extreme low temp start-up could easily be worth the additional investment of synthetic engine oils.

When I was working as an auto mechanic in the early days of synthetics, I saw many instances of moving to a synthetic in an automotive engine causing much more oil leaking.  Nothing catastrophic, but noticeably more leaks were common.  It seemed that those that didn’t leak before changing didn’t leak after.  But, those that did leak, would leak more after the change.

Like all things in engineering, it’s a cost/benefit trade-off.  For me, the additional cost isn’t justified in my usage, but I know it works well for many.  We’re still using dino oil in Dirona.  I changed the oil this weekend, warmed it up, and checked levels as usual.  The oil hardly showed any color (see below) — just what we like to see.  Whatever oil you chose, change frequently.

                                                –jrh