I know that the owners manual for my '05 calls for 75/140 syn, but I can get 75/90 syn at work for a pretty good price.

Should I even consider using it?

 

the 90 weight is for the front diff(4x4) and the 140 synthetic weight is for the rear diff with the Ford LS.

what brand of gear oil you getting for a great deal?
i recomend Amsoil or Mobile 1 (try Lucas as an additive)

 

I forgot to specify that I'm talking about the rear axle, 10.5 LS. And yeah I know that the manual says to use 75/140 in it.

I believe that the gear lube at work is Mobile in the 75/90 flavor. Should it even be considered for the rear diff (along with the Ford friction modifier)?

 

Over the years of the Sterling 10.5/10.25" rear used by Ford, I think they had a lot of pinion bearing failures when they first started using these rear-ends. I believe they were using regular 75w90 dino oil at first.

Since then, they switched to 75w140 full synthetic, and the failures went away.

I wouldn't stray too far from what Ford says to use, especially with the 140 vs. 90.

Unless you never tow, never carry a load, and do not have oversized tires, I wouldn't chance it.

 

90w will get too thin at high temps. There is a reason they spec 140w synthetic.