I picked up some axle shafts at East Coast Gear Supply (awesome shop, by the way!) and I asked them which differential fluid they use when they re-gear Jeeps, etc. He said every diff that leaves their shop uses Lucas 85/140 dino fluid. They've found the synthetic diff fluids do not last compared to the traditional dino oil. He said it's different for motor oil, but for their shop they stick with Lucas. He also said they don't get any special pricing; if they run out and need something quick, they can pick it up at the local auto store.