What gear oil to use?

[MorganMcCann]

Everything I see online says to use 80w-90, or 75w-140. I called ford and they said there is a TSB saying to use 140, however I did not see that tsb online.
I've got a 8.8" with a open 4.10.
Also should I use synthetic, synth-blend or conventional?

 

[exbass94]

I don't see any need for 75w-140. Use a 90 weight, such as 80w-90 or 75w-90. As for conventional vs synthetic, it's up to you. You don't really need synthetic, but you can use it if you want. If you haul/tow lots of heavy loads, it would be a good idea to use synthetic.

 

[MorganMcCann]

What exactly would going up to 140w do to the rear end?

 

[exbass94]

It probably wouldn't really do anything. It won't hurt it. It could theoretically reduce your gas mileage a little, since it's a more vicious oil. I just don't really see a need for it.

 

[SenorNoob]

I know my 8.8 has a tag that says "75-140 Synthetic Only". Right on one of the cover bolts. Came from a 95 Explorer and has 3.73's with Limited Slip. I'd think that would be the most important factor there. Non-Limited Slip axles should be able to use pretty much anything as long as it's close.

 

[gw33gp]

The tag on my differential say 'use 75W-140 synthetic only' but I think that is because of the Torsen L/S. My owners manual says to use something like 80W-90.

 

[4x4junkie]

My '99 Explorer axle had the 75W-140-only tag also (it was just a Traction-Lok L/S).

75W-140/85W-140 is good if you're in the hotter south (So-Cal, AZ, NM, TX, etc.), and/or if you do a lot of slow-speed rock crawling. In the colder north areas, stick with 75W-90.

Synthetic oils should run cooler than non-synthetic (less friction), which in theory should improve MPG (though probably not by anything real significant). I've also seen occasional wheelbearing wear on some of these axles that I wonder if synthetics might help reduce.

 

[MorganMcCann]

Well, I'm questioning all of this because one of my rear seals are leaking. And it is going to get fixed before winter. So, would the difference between 75w-90 to 80w-90 be a huge difference?

 

[4x4junkie]

Probably not.

I think it may be easier to find synthetic formulations in a 75W-90 (80W/85W-90 oils tend to be dino stuff, at least that is what I've observed in my area).

 

[MorganMcCann]

Finding 80w90 in synthetic is almost impossible, unless you want to pay a crazy amount.

 

[MorganMcCann]

http://www.amsoil.com/shop/by-product/gear-lube/synthetic-80w-90-gear-lube/?code=AGLQT-EA

Amsoil makes one, but still kinda high...

http://www.autozone.com/autozone/ac...il-80w-90-32-oz/_/N-255s?itemIdentifier=25555

 

[lvwill]

Recently had a very reputable shop in las Vegas regear my d35 and explorer 8.8 from 4.10 to 4.88 they used Yukon gears and install kits. They were very adamant about using only 75/140 synthetic in both axles. The use only Lucas products in the shop

 

[MorganMcCann]

I ended up buying valvoline syn power in 75w-140. I haven't had a chance to do the work yet, but I bought it all during the 30% off weekend for AutoZone employees.