I wouldnt worry unless youve been noticing/experiencing problems. Its good that youll be doing your own fluid changes from now. Its always best to do your own fluid changes. That way you know for sure its being done, & its being done correctly.

You could use
Front diff 75w-90, or 80w90.
Rear diff 75w-90, or 75w-140. If youd tow/haul heavy a lot, 75w-140 would be best to use.

Id just use 75w-90 in both diffs.

 

With that type of towing, I think I might opt for the 75w-140 synthetic in the rear and the 75w-90 synthetic up front.

I run Amsoil 75W-90 syn in both differentials. But, I don't tow anymore and opted for the lighter weight synthetic for fuel mileage...not sure it helped....but the synthetic has a longer drain interval. Mobil 1 syn is also very good in the same weights.

If you want to save a few bucks then you could run the SuperTech (Wal-Mart) or Travellers (Tractor Supply) brand synthetic gear oil. They meet the spec and are both full synthetic. Lots of options out there.

 

So how does he pull the diff. drain plug without dumping all the gear lube? And, what was planning on doing you you passed on the gear lube change?

Inquiring minds would like to know :idea:

DEWFPO

 

No- you likely don't need to worry. It's typical to see a bit of fine particulate in the diff fluid, especially when towing.

As long as you use a GL-5 rated lube with some FM, you'll be fine. The grade won't matter that much; anything from 75w-90 to 85w-140 and in-between will be OK. Consider getting a lube that is API J-2360 approved. Some reading you can do:
J2360: Lubricating Oil, Gear Multipurpose (Metric) Military Use - SAE International
SEA Automotive Gear Lubricants | DrivelineNEWS.com - Education & Tools | DrivelineNEWS.com
PRI Automotive Lubricant Review Institute (see the current listing; note that there are many conventional lubes on there and syns are not a prerequisite).

As for the FM, you'll need some in there. The factory fluid comes with FM, as does the service fuids from GM. Once you dump the factory fluid, you'll need to buy a product with FM included, or add a few ounces if it does not come in the fluid.

 

Recommended a tranny shop down the road to do the work.

 

Thanks everyone. Used 75/90 front and 85/140 rear.

Mike