I've recently noticed a case of vanishing oil on my 97 Grand Prix GT. 3.8l NA rebuilt with 40k on the engine since then. I use full synthetic Mobile 1 oil and I've noticed over the course of say, 2 months give or take I'll go through about half a quart or a little more. I have heard that these engines tend to just do that but I don't really believe that. I don't see any leaks (on the ground anyways) or smell any burning oil. Ideas?