My Avalon keeps fouling air filters with oil from the engine, to the point that oil pools at the bottom of the housing that holds the air filter. My mechanic checked and cleaned the PCV system but says the rings are bad and would require an engine rebuild. The car doesn't smoke like it's burning oil, though it sometimes smells oily and hot inside the car. There are 140,000 miles on the car and I'm wondering if the rings seem like the logical source of the problem.