I've got an '02 3500 4x4. In my truck, the front axle is always turning, even when not engaged, right? It's time to change the gear oil in both axles, and I'm going with synthetic.

I do a lot of summer towing in desert temperatures and on steep grades. So I'm thinking that the best gear oil weight for the rear is 80w-140. But I don't tow with the front axle engaged, and even when it is engaged I'm going at very low speeds through sand, snow, or rocks.

It seems to me that 80w-140 in the front axle would contribute to "drag" with the axle turning all the time. So I'm thinking I should put 70w-90 in the front axle and 80w-140 in the rear. Any problems with using different weights in these circumstances?

Thanks! I.