They charge way too much to change fluids, and I prefer to put my own in anyway. I changed the axles at 15k, should have done it at 5k, and used Mobil 1 75W90. At 30k I used Mobil 1 7W90 and 75W140 in the rear, and there seems to be less 'rumble' at the low end of the RPM range in top gear. The 75W140 is thick as well as expensive, so I might try a 50/50 blend next time.

 

Is it pure preference to run a synthetic over a good gear oil like Texaco's brand? These differentials don't require synthetic do they? If I run a gear "oil" I'll just have to make sure I run the friction modifier. The reason I ask is that I talked to the gentleman that owns Houston Ring and Pinion and he recomended running a natural gear oil as opposed to a synthetic gear oil. Any comments or suggestions are appreciated.

 

It may be pure preference, but unlike the engine's crankcase, syn lubes make sense in the driveline, no product of combustion contamination issues. Sure water or other contamination may occur, but being a much more thermally stable basestock, it should not break down as fast as conventional petroleum based lubes and allow for extended drain intervals.

The rear axle fluid on Dodge, Ford, and GM HD trucks now all come from the factory with synthetic. There's obviously a cost savings/benefit for them somewhere, or they'd be using the cheap stuff. Probably gets the vehicle out of the warranty period.

The future is always uncertain, but if you plan on keeping the truck a long time, invest in a good, well formulated synthetic for your own peace of mind. IF not, don't bother changing the fluid at all, it isn't going to do anything to increase the value of the truck at trade-in time.

 

I am almost positive my owners manual specifies for a synthetic gl-5 rated 75-90. Don't wan't to go out and check it, but if you are ever bored sitting in your truck, pop open your glove box and check you owners manual out!