amsoil 15-40, amsoil torque-drive (made for an allison). I run a fram filter, changed every 5k, add 1 1/2 quarts of fresh oil - working on 35 K no change! I send oil samples out every 10K, at 30 my report say 63% oil life remaining. I recently had a torque converter failure, so built the trans while it was down and let me say for 117K of whoopin' the internals were suprisingly nice. I firmly believe in amsoil - i picked up almost 3mpg by running both diffs, trans, t-case, and engine on it. Costs a little more but well worth it - Good luck with your new truck