Hello,
What is recommended maintenance schedule for the differential oil? What are you guys using as a replacment? It seems like Royal Purple is popular.

Jason

 

RP or Redline is what i'm going to use if I ever get to changing my gears.

 

The Synthetic Fluid can be pushed no-sweat to 100,000 miles. If it becomes dirty, then I would change it sooner, but as long as it looks clean, you should be fine for that mileage.
I believe the stock Ford Synthetic is very good. It does not have Friction Modifier, so if that is what you wanted, it is the only way to get it. If you have a limited slip, then more choices are available, but all other commercially available 75w-140s come blended with Friction Modifier.
For use with the TrueTrac, the Ford fluid is the best (and most quiet).

 

I have 41K on my car and it's the stock traction-lock dif.

 

The manual states that you need to add friction modifier before putting in the Ford fluid.
Other than not having to add it,are there any differences with pre-blended?

 

I think that is pretty much it. If you use the Ford fluid and you have a limited slip, then you need to add the friction modifier, just as you say. It seems like all the other gear oil blenders just add the friction modifier to the gear oil, so for those you would just pump it in, without having to mix in anything.
Also, please be sure to buy an oil pump designed to screw onto the bottle to get the fluid into your rear pumpkin without spilling. At $20+ per quart, you don't want to lose any of it.