Careful--not any oil
Jim Seamans is right, but be careful. • Yes, do use Yanmar 15W-40 oil, which is properly rated for the engine. I also learned at a Yanmar seminar that you can also use Texaco 30 wt if you can’t find Yanmar oil, but look at the container: the oil MUST be rated for API service and CD rated. This is very important since diesel engines develop much higher compression than car gasoline engines, and unless the oil is marked CD and API it will break down very rapidly and you could suffer major engine damage before you know it.
Also be very careful as you add new oil. Add 2 1/2 quarts and stop. Run the engine and stop. wait 10 minutes and check the level. If not up to the mark on the dipstick carefully add a few ounces, and repeat. Keep adding a few ounces at a time until the oil is at the mark. Do not exceed the mark, as you will get smoking and crud up the insides.
I strongly recommend that if you did not get an owner's manual and a Yanmar manual with the boat that you access the web sites and download them. And read them carefully--it could save you a lot of heartaches. Enjoy your 356--mine is a honey.