I doubt it's your piston rings, but it may be your valve guide seals and/or valve guides. How many miles on the engine? I don't think it's the piston rings because, when I had my engine looked over at 150,000 miles, the piston rings had only 4% leakdown.

It's probably oil seeping past the valve guide seals. Those are a pain in the arse to replace with the cylinder head on the car, and taking the head off is equally painful and expensive. If the problem went away with the thicker oil, just keep using it.