I have just purchased a 2001 2500HD w/8.1 , Allison 5sp. Auto .
Only has 12k miles on her. What kind of probs. can I expect? Are there any
performance upgrades to help w/fuel econ. w/o hindering the HP?

 

Welcome to the site!
Start with an intake, a programmer, exhaust, and maybe headers. Plenty of stuff out there for the 8.1L, even an intercooled supercharger kit!

 

That 496ci is a pretty good motor you shouldnt expect any major problems. Never heard of anything bad with the 8.1. Theres plenty of bolt-on's out there for that motor. You can do everything mentioned above and get a custom tune.

 

oil consumption, I'm not sure if it's a problem with a lot of guys, but a few have that problem. oh yeah, it sucks down gas like nobody's business