Synthetic.
Steve: The first thing. The purpose of synthetic oil is the fact that do not need to change it as often and it MAY provide better lubricating qualities. You just change the filter every 3000- 4000 miles. The oil only needs to be changed every 25k-50k. The problem with a diesel (as you know) is the oil looks dirty quickly. Next. Yanmar does not recommend synthetic. They now have endorsed Multi-grade (check out their web site). The idea that your engine is going to run any cooler is sort of a falicy. Unlike your automobile that only has an air cooled radiator, and you are running in traffic, up/down hills in stop and start traffic. The marine engine always has a supply of cool water running through it. You also do not want your diesel to run too cool (which I do not think synthetic is going to make any difference) for maximum operating efficency. The regular 30wt oil will hold up longer than you care about. You are in an area that the 30wt or multi-grade oil is not going to get too cold to easily start your engine most of the year when you are going to care about starting anyway. Most recommendations are that you change your oil every 50-100 hrs of operation or a least once year. So basically spending $2.50 on your oil vs. $10.00 does not really make much sense. You are really not gaining anything and Yanmar does not recommend it. There have been several discussions about synthetics in the past (look it up in the archives). The general consensus is using petro based oil in your Yanmar and changing it regularly is really the most prudent way to go. Spend the extra money on some good beer.
