I suspect the spark plugs are becoming fouled with oil and misfiring. Unburned fuel being sent out the exhaust because of misfiring spark plugs will destroy the catalytic convertor. Very expensive to replace. Things will start going down hill faster from here unless the situation is addressed. Using a high quality P.A.O. based synthetic oil in a 10W-40 viscosity may help the situation. High quality P.A.O. based synthetic oils burn much cleaner than old school petroleum oils do and can help reduce or even eliminate spark plug oil fouling.
One of my customers had a car that was burning a quart of oil about every 150 miles and fouling out a set of spark plugs every week, which was getting very expensive. We moved the engine up to AMSOIL 10W-40 Premium Protection synthetic motor oil - and the car never fouled a spark plug again for years afterwards. Oil consumption was reduced to about a quart every 1,100 - 1,200 miles, which also saved a lot of money in oil costs. We also used the AMSOIL Engine Flush to help clean out the engine and free up the piston rings from the petroleum oil sludge and goo that was making them stick, not seal and allowing oil up into the combustion chamber where it was burned and sent out the exhaust./QUOTE]
Great info here, thanks a bunch! I have a 2002 Corolla and was looking into doing an engine flush after reading this. I read - "In very old cars, however, the engine flush might clean these piston rings, valves, and other parts a little too well. Sometimes, the gunk acts as spackle in non-metallic parts like rubber seals that have cracked with age. Cleaning out the gunk exposes those cracks, and the weakness of those parts becomes apparent." I am wondering if you have found this to be true and if I should still go for it?