'02 F250 7.3L Diesel with oil in Coolant overflow container?
 

I have a 2002 F250 7.3L diesel and I just bought it a couple months ago and have less than 2k miles on it by me. It has 140k on it. It is beginning to show oil in the coolant overflow/expansion tank(the clear plastic tank). Any ideas on what could be causing this problem? I haven't seen any coolant signs getting into the oil (chocolate milky baby-poop), so I'm at a loss as to where this might be coming from. I haven't noticed and performance issues per se. Another problem I've noticed is that the tranny temp jumps up when it down-shifts when pulling, and then it might go back to normal after a few minutes or it might go completely to the cold mark. Otherwise the truck runs fine, and I bought it because I've heard these engines were bullet-proof. Thanks in advance for any help you can offer me on these issues.
--eric