Chinese M20 compression. "Won Is Lo"

[Rustam]

Helped neighbors kid do compression test on M20. Five 150s and Won Is Lo at 60 psi. Few good shots of Marvel Mystery Oil and Won Is Low pumped out solid 175 psi. Won Is Lo is #1. Recent head gasket. Runs well so low cylinder a surprise.

What causes one cylinder to be Lo if oil test shows valves OK?

I think the rings may be bad. The oil poured into the cylinder creates a temporary seal between the piston and the wall. Having bad compression without oil and good compression with oil signifies worn piston rings.