I used Amsoil "Severe Gear" 75/90 in mine. I used it before in my 04.5 and again in my 06 that i traded up to and had good results. Just getting rid of the old stuff from the factory is a step up in the first place. Amsoil also carries a 75/110 and a 75/140 if you have the need for something with more protection for heat and wear. If you have any chance of dipping your pumpkins under water, it's always a good idea to drain and refill anyway. Water will kill the benefits of any product if it's allowed to mix and seperate with the fluid you choose.
Check out this study that was posted on the Amsoil site - http://www.amsoil.com/lit/g2457.pdf
- it's a comparison of several different brands of gear oils. There are some that excel past Amsoil and some not so much. One of the items that were important to me was the cold tests since it does dip down below zero a few times a year and I work 50 miles away from my home. It's a happy medium with good coverage for all seasons and is price is comparitive to other brands. With my preferred customer card, i get it less then what's stated on the sites and if you know of a dealer (online or not) you can get "sponsored" to get your own card. I was lucky enough to get under one of the guys on the cummins forum i belong to.
Anyway, that's my 2 cents. Good luck in your research.
Where we've been:
2000 Rockwood Roo M-18 Hybrid
2006 Dodge Ram 2500 Auto 5.9 Diesel
Gone but not forgotten -1986 Rockwood 1003 PUP