[jfreddie]

The lack of any piston damage above the oil ring says there was no excessive combustion temp or detonation.

So if the oil ring was the problem, that could only have happened through things like too tight end gap and cyl too hot (lack of water), or deformed ring at installation.

For no more damage than there is I would put a light hone on the cyl wall, get a new piston and ring set, measure it all as previously discussed, and reassemble and run with a good 10w30 and confirm radiator full.