I just recently purchased a 2000 Camry 4 cyl engine with 200,000 miles on it. The Engine runs great, uses no oil but has one problem. When the engine is cold it is very quiet, but as it warms up, I hear a loud knocking(tapping)sound in the valve cover area as the engine reaches around 2000 rpm. I contacted two shops in town and the Dealer. Both shop recommended running a synthetic oil first, to see if would help free up and clean this engine. When I contacted the Dealer and ask how much to adjust the valves. Their answer was this "normally the valves never need adjusting and the cost would be as much as a used engine." My question is this, what oil and weight would be the best, also could the barrel lifters on this engine, maybe not flowing as freely as it should and at higher rpms, making the gap farther apart at times?