(blow smoke)? Have you had it checked for leaks? Z's often have a rear main seal leak, which is hard or impossible to detect w/out dropping the transmission. Some other leaks, depending on where they are, may burn off the seepage and you may not be able to tell (give it a sniff :) Barring the obvious, leak-wise, have it dye tested to see what happens. Hope that it's leaking and not burning. For me, switching helped. I always run synthetic (Mobil 1). I switched to 10W-30 and have not lost a thing. Good luck!