Rear diff fluid

[KYLE2289]

I have tried to search for the correct fluid but all I get is 75/140,80/90 and a few other options can someone help with this it does have lsd.

 

[Maniak]

The LSD means you need friction modifier.

Some Diff fluid has it already, others require you to add that small bottle of friction modifier.

As for which viscosity, I've run synthetic 75w/140. The current gears in the rear have over 210k miles on them, with a Detroit locker and last time I had it open I was just starting to see some wear on the face of the gears.

As for brand, I've always run Mobile 1 synthetic (except the time I had to throw some in due to a axle seal leak).

I've seen people run the lighter stuff (75w/90) without issue too.

It's not quite as contentious as the dino vrs synthetic debate but the diff fluid viscosity doesn't really have a consensus.

~Mark

 

[ron72gt]

Iv'e always used 85W140 convential gear lube with friction modifer from Ford in my vehicles.

 

[chefduane]

My 98 has a 3.73 LSD and I used Valvoline 75w-140 Full Synthetic Gear oil and Sta-Lube Equa-Torque Friction Modifier (meets Ford spec). No problems so far.

 

[natenkiki2004]

It depends on how you use it. If you want to offroad a lot or tow a lot or you're in a hot climate and you run high speed long highway runs, you're probably better off using the 75W-140. However, that's 2-3x the price of 75W-90.

You'll find all kinds of information out there but I'll try to sum it up since I researched plenty before changing the fluid in mine;

These Explorers came with 80W-90 regular oil from the factory. Somewhere in the late 90's Ford changed recommendation to a heavier oil (I'm not sure what exactly it was, if it was synthetic or regular but I'd guess 85W-140). This change was primarily for noise reduction (not chattering but general noise). You gotta keep in mind, these Ford 8.8's are well built but they're on lighter vehicles that aren't meant for heavy use.

Personally, when I got my Explorer, I sucked out (not drained) the rear diff fluid and put in fresh regular 80W-90 and a bottle of the Motorcraft LSD additive. Used that for 6 months or so, just as a flush. About a month ago, I popped the cover and thoroughly cleaned it out inside and refilled with 75W-90 Mobil 1 Synthetic, no additional additive. In my case, most of the time the rear cargo area is empty and I'm not hauling. I may drag a trailer around every so often and offroad a bit but I kept everything mostly stock, it's primarily a winter transporter.

Anyway, through the process of changing fluids from whatever the previous owner had to the 80W-90 Walmart special to the 75W-90 Mobil 1, I never once had a difference in noise, chattering or any abnormal behavior, and LSD lockup has always been the same.


Honestly, the simple fact that you're even changing the fluid will extend the life drastically. Get some quality synthetic oil but don't overpay and don't overthink it. It'll be plenty happy with what you choose.

 

[RangerX]

I've run 80/90 in mine for 20 years and 300,000 miles now. Is there a better weight? Probably. Is there anything wrong with 80/90? Nope!

 

[RangerX]

Also, you can skip the friction modifier for the limited slip. All it's meant to do is make the LS less noticable to soccer moms by helping it slip more, and grip less. But yours is 20+ years old and most likely completely ineffective, so to not add it would actually help!

 

[natenkiki2004]

Also, you can skip the friction modifier for the limited slip. All it's meant to do is make the LS less noticable to soccer moms by helping it slip more, and grip less. But yours is 20+ years old and most likely completely ineffective, so to not add it would actually help!

I don't know that I'd go this far to NOT add any. You should see if your limited slip even works, get on some gravel and peel out. If it does work, then the additive included in modern synthetic fluids is plenty fine. If you go for regular oil, I'd suggest adding a bottle of the additive. My clutches have never been replaced and laid 2 long tracks nicely in snow with 80W-90 and a bottle of additive.

Yes, the additive helps slip more, that's why I ignored the advice to add a bottle in addition to the additive included in the synthetic oil. But, as with many things, there's a balance. You don't want it locking up and chattering around corners on pavement. That'll put unnecessary bind on the axle and clutches, wearing them out quicker. Too much slip though and it might not lock up when you need it. I've put a few hundred miles on since changing to Mobil 1 synthetic without additional additives and haven't heard a peep from the rear.

 

[chefduane]

I don't know that I'd go this far to NOT add any. You should see if your limited slip even works, get on some gravel and peel out. If it does work, then the additive included in modern synthetic fluids is plenty fine. If you go for regular oil, I'd suggest adding a bottle of the additive. My clutches have never been replaced and laid 2 long tracks nicely in snow with 80W-90 and a bottle of additive.

Yes, the additive helps slip more, that's why I ignored the advice to add a bottle in addition to the additive included in the synthetic oil. But, as with many things, there's a balance. You don't want it locking up and chattering around corners on pavement. That'll put unnecessary bind on the axle and clutches, wearing them out quicker. Too much slip though and it might not lock up when you need it. I've put a few hundred miles on since changing to Mobil 1 synthetic without additional additives and haven't heard a peep from the rear.

I think I'm with nate on this one. Every thing that I have read (on the interwebs no less, so hey...!) says the 3.73 lsd needs the friction modifier. Ford spec's using it so I did. Could you get away with NOT using it? Probably. But the cost of using it is negligible so what the heck. Here in the DFW area it gets hotter than >heck< in mid-summer so the 75W140 is warranted on my 50 mi round trip each day. We can get minimal snow and ice in the winter (not much) but when a storm does happen folks here freak out. It's nice to have the 4x4 w/ the lsd and not have to worry about sliding off into a ditch (which happens to folks with regularity when we get even a hint of real weather.)

And I agree completely with the fact that just changing the diff oil is significant. The vast majority of Ex owners have never even done a siphon/re-fill much less popped the pumpkin and done a full flush.