I am thinking of changing the fluid in my differentials on my '97 Dak Sport 4x4 with 5.2 auto trans. Factory fill is apparently 80W-90 with added friction modifiers. I assume the front takes the same weight? I was thinking of possibly using 75W-90 synthetic plus friction modifiers. Will this hurt anything or would I be better to stay with the 80W-90? I have 91,000 miles on the truck, some say to never change the gear lube, only to top it off. Change it or let it go? I have never changed the gear lube in any of my vehicles in the past. Puzzled? Thanks for the help in advance.

 

i changed mine not too long ago, i used royal purple 75-90. its a good idea to have fresh fluid in there, and inspect the gears. so far no problems with the 75 but its so close to the 80 its not much difference.

 

75w90 will be A-OK, so long as you add the required friction modifier called for.

 

what rear? the corporate 9.25 calls for 75W-90 even 75W-140 if you tow a lot in the rear and only friction modifiers if you have a limited slip...the front however which is the same on all 4x4's takes 80W-90 and I changed both of mine to synthetic with over 130k on them and had no problems well except for the fact that before I put it in the first time I had to get it rebuilt...

edit: checked my FSM and the 8.25 takes 80W-90 so like I asked depends on the rear and you only need friction modifiers if you have a LSD. also a lot of synthetics come with the friction modifier in it but I still put it in my rear when I switched over to amsoil to be safe

 

RP has it already added.

 

I read somewhere that the different viscosity's have different additives or formulas? Forgot to mention that. Does anyone know if this is true?

 

3.90 rear end with limited slip

 

i put 75W-90 amsoil with some friction modifer in and have been happy/no problems as well as 80W-90 amsoil in the front so i'd say if your **** about fluids and stuff do it, it wont hurt i had about 30-40k more miles than you and did the synthetic switch with no probs so far and i still get 19.6 MPG with my tires and mileage on the HW

 

depends on what you get. when I got my amsoil it said if it chatters without modifier add it but I just decided to get it and be done with it to start instead of having to get it later, it wont hurt it

 

I was thinking of Mobil 1 synthetic, even though it has modifiers in it, the web site said I would have to add more. I just couldn't find anything about using the 75W-90 instead of the 80W-90. I just figured if would flow easier when cold, but not sure about it because of the pressures it would be under.