Rear Differential Fluid
 

I have a 2012 3500 dually and getting ready to replace my rear brake rotors. I expect to lose a small amount of fluid when I pull the axles and I'm not sure weather to top off the fluid with synthetic or non synthetic oil. I'm just sure its going to be 75w-90.

Always use synthetic oil in HD differentials and when used for towing. Synthetic does not break down under high temperatures.

Since RAM started using AAM axles back in 2003, they have all been shipped with GL5 75W90 synthetic.

Pardon my lack of dually knowledge, but you have to pull the axles to replace the rotors?

I believe on a 3500 you do. 2500 and below, no.

Here's a random video of it being done on a little older model

Yes they need to be removed.Then the hub assembly needs to be pulled off and the rotor has to be unbolted(8 bolts) from the hub.
First time for me:o

It's not a big deal. There's no gear setup involved. Take pics as you go. Keep things in order and clean. Watch the vidja above.

I’m going to drain the FF @ the 15K mile mark & add a Mag Hytech Cover & refill with Red Line 75W-90 & mix in 1qt of RL 75W-140.

Would filling a 2017 3500 chassis cab diffs with 75-140 hurt anything? We have a bunch of it for the old trucks laying around, trying to keep things simple....

I think 75/90 is entry level. I use syn 85/140 in all my trucks for heavy towing. That's the only weight that keeps the metal off the magnet, for me. I figure mpg every tank and no decrease form the heavy stuff. Craig

Thanks, I’ll remember the “metal off the magnet” comment for a long time.