Yes, the compression pressure is less in the gasser, but the peak cylinder pressure is still about the same. You're taking the same torque and power from the same geometry, so the pressures must still peak in the same area. The engine tolerances are still the same, and they want the heavy oil.
Granted, I do use the 15W40 for much the same reason you do. I have it. It's easy to get, and generally just as cheap as trawling around looking for a good weight S rated oil.... In my area, it would not be easy to get a gas engine rated oil nearing this weight. Most everything is 10W30, and I'd be uncomftorable with that around here in the summer, as moderate as the heat is. I do use it in the cold starters in the winter... Even the CI-4 rated oil is rated for SH or thereabouts too.
Whatever works, and make sure it's got oil. That's the main thing.
Rod
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