I have an 03 Frontier with 3.3V6, auto, 4wd with 165,000 on the odometer. I've replaced both valve cover gaskets, and other stuff that are non oil related. Now my mechanic tells me the oil leak is coming from the rear main seal and possibly the head gasket. Says he can't tell if the head gasket is leaking for sure till he gets the tranny down to replace the rear main seal. Anybody with close to the same mileage had this problem? I mean if I have to pull the engine to replace a head gasket, then I'm kinda considering getting a JDM used engine with low mileage off ebay and putting that in. I can't imagine at that point the price would be that much different. So next question has anybody ever bought one of these engines and had any success with it? Help please.