My 2002 Nissan 2.5 Altima has oil pooling on top of one piston and burning oil out the exhaust at the rate of 2 quarts per 15 to 20 miles what could this be. No water in oil or oil in water.
if oil is getting into the combustion chamber, it gets in through the rings or valve guides- since you say it is "pooling" (how do you know?) I would guess worn valve guides-
I know it was pooling because I pulled number 4 spark plug and I could see it pooling on top of piston and i removed valve cover and could see it also new valves have been put in but not new seals by prior owner
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