I have a 92 GMC 3/4 ton 4wd. There is a constant smell of gearbox/hypoid oil in the passenger compartment that I can't get rid of, and I can't identify how it is coming into the interior. The engine compartment is clean, and free of oil, and I have tried moving the breather hoses around on the transmission, transfer case, and differentials. The smell is very noticable while driving with the windows open, but hard to locate when the vehicle is stationary. Anyone else run into this problem?