Gear Oil Questions

453 Views 10 Replies 8 Participants Last post by RustProof, 5 d ago

BakInBlak

Discussion Starter · 14 d ago

Hey all,

I've tried searching through the forums but can't find an answer for this. My 1990 5.8 E4OD is spilling gear oil out of the rear axle breather. There's no tube on it so I suspect water got in while driving in the rain.

I'm going to change the fluid and install the breather tube. However, I have no clue what oil type I need and am not sure if it's LS or open diff.

Can anyone offer any details on this? Thanks!

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BigBlue 94

· #2 · 14 d ago

BakInBlak said:
Hey all,

I've tried searching through the forums but can't find an answer for this. My 1990 5.8 E4OD is spilling gear oil out of the rear axle breather. There's no tube on it so I suspect water got in while driving in the rain.

I'm going to change the fluid and install the breather tube. However, I have no clue what oil type I need and am not sure if it's LS or open diff.

Can anyone offer any details on this? Thanks!

Check the sticker in the drivers door jamb. There is an AXLE slot with a 2 or 3 digit code under it. If it is 19, 18, 15 etc, its likely not a LS. If it is H9, H8, H5 etc or H92... Then ots likely an LS. If you open it up and look at the small gears in the middle of the differential, and there is a stiff S shaped spring in the middle, its an LS.

Lucas HD 80w90 is a good gear oil.
I prefer a high end oil by RedLine called heavy shockproof, but it isnt necessary on a mostly street driven rig.

Whatever you choose should be rated GL-4 or GL-5. 75w90 or 80w90 is what you need.

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BakInBlak

Discussion Starter · #3 · 13 d ago

Thanks Big Blue. I've got an H9 code so I'll get a limited slip oil just in case. I appreciate all the help

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fordbronco1995

· #4 · 12 d ago

Also i know most say it isnt needed but if it is indeed a LS i would add 4oz of motorcraft friction modifier with lucas 75w-90 synthetic gear oil

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BakInBlak

Discussion Starter · #5 · 12 d ago

fordbronco1995 said:
Also i know most say it isnt needed but if it is indeed a LS i would add 4oz of motorcraft friction modifier with lucas 75w-90 synthetic gear oil

Does the synthetic really have any benefit? I've heard about people having issues with it in other vehicles. I just bought LucasOil 80w-90.

As for the LS additive, is there a certain rating I need (ie, XL-3)?

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dash_cam

· #6 · 12 d ago

i bought some gear oil from Peak, and it contains friction modifier in it. i called and spoke to the tech support and they gave me the % of friction modifier in it. i don't remember the value, but it was appropriate for the bronco.

i changed differentials and no longer need the friction modifier. now i use regular valvoline w/o additives.

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fordbronco1995

· #7 · 12 d ago

BakInBlak said:
Does the synthetic really have any benefit? I've heard about people having issues with it in other vehicles. I just bought LucasOil 80w-90.

As for the LS additive, is there a certain rating I need (ie, XL-3)?

the synthetic can go longer between fluid changes. Also resists breakdown from heat.

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Kingfish999

· #8 · 12 d ago

majority of synthetic gear oil contains friction modifier. of course theres exceptions but most do.

generally synthetic has always been regarded as better. supposed to tolerate higher pressure, higher heat, and be more slick. however i have seen some places claim opposite

ECGS Gear Oil Recommendation

ECGS recommends Dino (Non-Synthetic) 85w140 Lucas oil with a Ford or Dana limited slip additive for clutch style posi's. The gear oil question is an age old debate that comes down to opinion. We base our opinion on differentials we have seen. Scientifically it has been proven that when measuring...

eastcoastgearsupply.com

alot of newer Ford axles come with a tag that specifies to only use synthetic. for example almost all the newer Sterling (10.25"/10.5") axles say to only "75w140 synthetic oil only". even some of the diff covers will be stamped it

Also if you use the truck for harsher operation like towing or offroading. i do tend to use 75w140. Funny thing is that the Ford Explorers using the 8.8" axle with 4.0 and open diff use 80w90. while limited slip or 5.0 use 75w140. so yes the 8.8" is fine with 75w140 for heavier use. 80w80 is more efficient tho for daily driving

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Guns & Horsepower

· #9 · 8 d ago

If you live in the Southwest, 85w-140 isn't a bad call. The driveline shops here in SoCal recommend this weight if you do any kind of off roading or towing. Towing across the desert when it's 110+, you'll want the heavier stuff. OEM's specify synthetic to get their Cafe mileage up 0.010 percent, and it has a longer (100k?) service life, allegedly. I run an Eaton Tru Trac locker in the Bronco and my '17 F-250, and Eaton specifically states to not run synthetic gear oil. It can cause the locker to not "lock" properly. The same might be true for a LS diff., although newer LS diffs specify synthetic. Common sense says super slick, and clutches don't play well together though.

I run Lucas 85W-140 dinosaur gear oil in the Bronco and F-250, and change the diff fluid every 10K. The lockers work as advertised and never an issue. Stuff smells like chit! I hate the smell, but the diff likes it.

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CrazyBRONCOguy

· #10 · 8 d ago

I think the key is like mentioned above... It matters more that it gets changed, then what your actually using (assuming you put friction modder in it if it calls for it)

I run 75-140 amsoil in all my own stuff and have never had any issues.... I have noticed a 1-2 mpg increase in the more street friendly stuff I have after switching (diffs, trans, tcase, engine) may not get same result in Bronco tho.

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RustProof

· #11 · 5 d ago

BakInBlak said:
Thanks Big Blue. I've got an H9 code so I'll get a limited slip oil just in case. I appreciate all the help

Recommend you get Motorcraft limited slip additive. I used some whatever-brand from the parts store once and the shudder in the LSD of my yukon 9" LSD was crazy - felt like the modifier made it worse! 😂 The motorcraft stuff only costs a few bucks and is well worth it IMO.

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