Tom,
As a temporaty fix, you might try Engine Restore. It's in a silver can at the auto parts store. You don't have to put the whole can in. Just enough to stop the problem. I once ran a 1000 mile rally with an engine that had way too much blow by. The Engine Restore got us through just fine. When I pulled the engine apart, I found a broken second ring on number 8 piston.
I would avoid STP or any other
oil thickener. They make the engine parts very sticky when you do take it apart.
Paul