the 15w/40 is usually certified for both gas and diesel, the S followed by a letter is for spark ignition (gas), the C followed by a letter is for compression igition (diesel)
I used to use the Delo straight 30 or 40 and that at one time was certified for both, but about 10 years or so ago, I started to see it only certified for diesel, probably doesn't matter much though.
The 15w/40 is a bit cheaper than the Merc/Quicksilver oil, I haven't seen tests to show if one is better than the other, its really the owner's choice, I go by the oil pressure reading I see after a run on plane, as long as the oil pressure is not dropping below 20 psi at slow hot idle, your oil is not shearing down enough to cause a problem. Normally mine runs between 40 and 60 lbs, even though the short block is original from 1988.
I would not use car based 5w--anything in a marine engine, unless the manufacturer specifically recommends it.