Differential Fluid Choice and When

[rbejexp]

I am curious on the cost of the Motorcraft 75W140 Synthetic, in comparison to others like Mobile 1. I know Mobile 1 is about $17 for a one liter bottle (autozone). There is a local guy that is sell a couple of complete kits to perform a change of differential fluids along with a RTV sealer and friction modifier. He is offering Motorcraft 75W140 (4 quarts) Friction mod., and RTV Gasket, all for $30.00. Does this sound like a good deal? He mentioned he was an ex Ford service guy, sounds all legit, but I woundering if there is a far greater advantage to using Mobile 1 over Motorcraft?

Also, there seems to be a question of whether to change the fluid out, since some say that Ford does not mention changing out the fluid in the manual this means it does not require service. I have a 98 XLT 6V SOHC with 70K. Should I change out this fluid to be on the safe side or leave it alone until I here of problems?

Thanks!

 

[rbejexp]

bump..any comments? - Thanks

 

[ma96782]

Comments........OK, here goes:

1) Differential fluids change interval.....for me 100K miles.
2) Syn fluid.......yes.
3) Some syn fluids DO NOT need the additional "friction modifier."
4) You only need the friction modifier, IF you have the LSD rear end. Other than that it's a waste.

Aloha, Mark