To understand the difference between auto and marine use, think of it this way. Marine engine do not have ability to use different gear ratios in the transmission to make the load easier on the engine. They can never coast or loaf down the road at 1700 rpm like the Chrysler Hemi in my Jeep can. Imagine if your truck had only 2 gears and your highway rpms were between 3600 and 4000 rpm all the time. Now while 4 stroke outboards are designed to run that fast constantly, I do not believe the old school Chevy small blocks were. So I have said and still will say, that Merc and Volvo did not recommend conventional lower vis oils in their engines, yet people feel the need to question what engineers have recommended and specified and will try anything just not to buy the 'approved' oil. Everyone wants to experiment and that's understandable but I would also say this: if you do that, then do an oil analysis to see, if that oil really stayed in grade. If so then fine, if not then well you learned that the engineers know more than we do!
PS I have done 2 oil analyses on my old '88 4.3, one with Chevron Delo 400 straight 40 and one with the Merc/Quicksilver 25/40. They both held up well but the Merc oil held its grade better believe it or not.
Another data point. People think that the NMMA spec for corrosion resistance for the marine engine oils is a bunch of BS. Well all I can tell you all this:
Last summer my engine blew both head gaskets and I had salt water in a cylinder, probably for about a week before I figured out what the cause of my reluctance to start problem. I blew out the water, fogged it several times and drained the block and manifolds not knowing yet where the water was coming from. Did more tests when I got the boat back home and confirmed it was not a leaky manifold but blown HGs. When I took it apart, there was no rust at all on the cyl walls, cam, lifters, rocker arms, etc. And this engine has been used in salt water at least 15 years. The only rust I found was a tiny bit of it on one of the pushrods, surface rust and some on the combustion chambers of the heads. The motor oil looked milky but I have to think, it being salt water, that the additional corrosion resistance in NMMA certified oils do help.
One last point:
1 check the amount of ZDDP in the lastest formulation of Delo and Rotella, its not much more than the gas only motor oils, it used to be but that was changed.
2 all the worry about ZDDP may be irrelevant, unless you have a Chevy small block older than 1987. Why, well the roller cams and lifters were adopted in that time frame, my old '88 has a roller cam. The concern over wear applies to flat tappet cams, not roller lifters. I can say that when I pulled out all the lifters, the cam lobes had very little wear as did the lifters, they looked almost new. So if you have a roller lifter cam you do not need to worry about the ZDDP issue, at all.