While under by truck today (93 F250, 5.8), I noticed an oily residue on the underside of the body. It appears to be directly above the small (1" diameter) pipe that runs from the cat back toward the motor. The residue appears to be blowing from somewhere, presumably the pipe coming from the top of the cat.

Any ideas?

 

some drippies from the transmission? that would be the only thing even close to your cat converter with a significant amount of fluid of any kind. other ideas?