Ray,
In the C36 Owner's Encyclopedia (on this website), there is a complete cross-reference chart for oil and fuel filters.

My engine has been out of the boat for two years now and I'm getting it ready to be put back in soon. Replacing all the hoses and relocated the oil drain further back on the oil pan to get all the oil drained when changing it.
When I went to buy oil at Wal-Mart it got very confusing to which one is the best. Most say they are for gasoline.
The owners manual for my Universal M-35 says to use SAE 30 HD. (CD) or 10W40. But which brand should I get tho?

Hard to go wrong with Shell Rotella-T. Choice of farmers all over Peterborough County, and probably elsewhere. Everybody has their favourite though.

I've used 15W-40 for a lot of years in my farm tractors and they work far harder than a boat engine does, never had an engine problem.

I am not a fan of straight weight oil (eg 30 weight) , multi-grades (15W-40, 10W-30, etc.) are so much more friendly to a cold engine.

Raymond - I have M35B. I use engine oil - Standard Oil Delo 400LE 15W-40 CJ-4/SM for diesel engine About $18.00 for 4 quarts. The oil filter is WIX 51365 at $ 9.00. Both are available in any auto store. The engine capacity is 3.4 Quarts of oil and manufacture recommendations is to change every 100 hours.

Ray
The oil for Diesel engines should have an API rating on the container with the letter "C" such as "CF".

Recently I spend many days discovering the correct type of oil that should be used in a diesel engine. I am including a PDF file that lists the various API specs for engine oil. On the top of the list is CJ-4 for Diesel engines. It is not clear to me if the marina that I am in is selling diesel #2 with ultra low sulfur. At the local gas stations the #2 diesel has, as little as, 150 Parts per million ( PPM) - this is clearly labeled on the pump. Diesel #2 has 500 PPM.

I went with Shell Rotella T SAE 15W-40. Not what we used to use but the guy who was helping me with a few boat projects used it and said it was a good enough choice.
It's hard making these choices when my husband knew the "right" ones to make. Of course he had his ways which may not have been other peoples ways. It all seemed to work out tho.
One more project crossed off my long list of to-dos.