Type of oil used for diesel engine

Look on the back of the container. There'll be a circle with the grade in the middle and the service designation around the top:



Anything with a Cx listed is rated for diesel engines. Each time they revise the service designation they increment "x". CD was introduced in 1955, and has been superseded by CE, CF, CG, CH, CI and now CJ. Each oil in the sequence is an approved replacement for those before it.

If you can't find 10W30 easily with a Cx designation, look for a straight 30 weight. The 10W part means it's a winter rated multigrade oil. Since you're probably not going to be using your boat in freezing temperatures, the cold weather properties of a 10W30 aren't needed.

Tim

It is best to stick with an oil than switch between paraffinetic and synthetic brands. Elastomers are compounded with base polimers, ie nitrile, hnbr, sbr, etc and additives, diatomic , clay, carbon black, minerals and plasticizers, oils and steric acid, waxes......and of course the catalyst or curing agent sulpher, peroxide, platinum, etc.. The elastomer will come to an equilibrium where the plasticizers are leached out of the compound and will be replaced by some of the fluid being sealed. This is most prevalent when sealing hydrocarbon fluids. If you keep the same fluid type, no issues, the elastomer is in equilibrium. If you change to a different fluid medium the elastomer can swell or shrink due to coming to equilibrium with the new fluid. Excessive swelling results in catastrophic failure of the seal, shrinkage can result in brittleness and leakage. Ammines are in a group of additives that are deliberately added to oils to control swell the seals to stop leaks. They work for a short period of time only to delay a necessary reseal cost.