I don't think the rings are stuck if the engine wasn't stuck.
Doubt any thing poured in will help at this point.
If it's got 20 PSI oil pressure warm and revved up, that is good!
Most barely move the gauge at idle, and will run that way for years.
But, if you go in, open the oil pump and look it over. They are known to break where the driven gear attaches to the shaft.
And the governor issues. Even if it works it needs to be looked at just to be sure it's not failing.