I stick to the 5000 routine regardless of how long some say synthetics last. When you go to do the rear diff make sure you look at the forum for suggestions. It's an easy task and the forum pro's give great info. I also added Purple Ice, another Royal Purple product, to my coolant and my rig runs cooler even with the A/C blasting in the Texas heat. I plan on having this baby for a while so I'm going to make sure I take good care of her. Hey while your under your ride take a look at the universal joints. Might be a good time to zap some grease in the fittings too. Have fun! :bandit: