Not all oils are equal. There are many oils out there that are made only for cars or Spark ignition and do not have the best additives for Compression ignition engines like a diesel. Every container of oil should have an API (American Petroleum Instutue) seal along with the oils ratings. Ratings like Sxx are for Spark ignition while a Cxx rating would be for a diesel or Compression engine. Many oils have no 'Cxx' rating at all so beware of these.
I like to use Rotella T 30 weight oil in our engine which happens to have spark plugs. Rotella T is a good choice for both diesel and gas engines (check its ratings). If you are planning on using your boat late into the season the 30W oil might be the better choice for cooler temps.
Change your oil often. It is cheap insurance.
08-01-2010 02:42 PM
It really doesn't matter. 30 is probably recommended for cool temperatures, 40 for hot. A boat doesn't really see either. Cold starting is one reason to use 30, but cold usually means below zero. Hot is only a problem for air cooled engines, not an issue on a boat. See what the manual recommends for the transmission so you can use the same in each, unless you have one of those odd trannys that uses transmission fluid. If your engine uses a little oil then I would use 40, otherwise 30.
08-01-2010 02:28 PM
It is a 1985 with light to moderate use.
08-01-2010 02:25 PM
30 should be fine unless it is an old engine with lots of hours on it, then use 40. I use 15-40 I think but it is recommended in my manual. There is probably little advantage to a multi weight oil in boat engines. The transmissions want straight weight oil anyway. Just use Delo or another Diesel approved oil. Use the best filter you can get too.
08-01-2010 02:14 PM
Recommended Oil for Yanmar 2GM20F
I'm getting ready to do my first oil change on my Yanmar 2GM20F. What is everyone's choice of oil? I'm on the Chesapeake Bay, according to the manual Yanmar recommends a 30 or 40 weight oil. Looking for for recommendations from the community.