I searched Priuschat and looked at the TSB. It sounds like it's the pistons and piston rings. I'm sure they are expensive to repair. I've been looking at options and I found some ideas.
1. Switch to a thicker oil, like a 5w30. This is rumored to cause a drop in MPG. Can thicker oil cause a drop in MPG? Will thicker than recommended oil damage the engine in any way?
2. Use Cataclean in the fuel. The post said "This product cleans catalytic converters. It has a side effect of cleaning the carbon deposits in the engine, intake manifold (from the injectors into the valves), and the exhaust manifold. This will erode the ring deposits from the combustion side. Also, use Motor Flush in the oil at the next oil change to clean the rings from the oil side."
Is there any truth to #2? Sounds simple to do. Maybe too good to be true? Will this method work on the piston rings?