I've been changing Rear fluid every 25K and Front every 50K since adding the Dynatec hubs. 4 rear changes, 3 front changes. I use 75W90 in both ends (I don't tow much except a occasional IHC Scout... and I'm between Scouts right now). So far I've used Royal Purple twice, Redline twice in the rear. One Royal Purple and two Redline's in the front. I actually have Amsoil 75W90 in the garage right now for the next change. I can't notice any difference in milage or noise and the oil looks, and smells, new at the oil change. I think any premium synthetic gear oil is fine. How often are you guy's changing yours... I think every 25K is way too often!