The best oil for an 80’s 7.5 differential

[Mike Tonon]

Which oil is it?

 

[sgtsandman]

Any name brand that meets the specifications in the owner's manual will work just fine. If you want to splurge a bit, then the same specification in synthetic.

 

[Dirtman]

If you're rebuilding the rear end, how well you build it and the quality of the parts will matter waaaaaaaaaaay more than what oil you use. You can put the worlds best oil in it but if you didn't shim it correct or used cheap bearings it wont last anyway. A well built axle with quality parts will run effectively and last a long time on any gear oil. If doing it yourself, buy a good service manual first and watch a ton of videos (youtube is no replacement for a manual). Oil cant fix bad work.

 

[Mike Tonon]

Any name brand that meets the specifications in the owner's manual will work just fine. If you want to splurge a bit, then the same specification in synthetic.

They were recommending 80w90 conventional. But in many cases, now recommend 75w140 synthetic. Why not 80w90 synthetic?

 

[Dirtman]

They were recommending 80w90 conventional. But in many cases, now recommend 75w140 synthetic. Why not 80w90 synthetic?

I may be old school but I've never seen 80w90 synthetic.. synthetic 80w90 is 75w90.

 

[Mike Tonon]

I may be old school but I've never seen 80w90 synthetic.. synthetic 80w90 is 75w90.

Why? Why are they getting complimented, here? That’s rather annoying.

 

[Mike Tonon]

So would you PRE-FER 75w90 synthetic or 75w140 synthetic? Or if I found 80w90 synthetic, what would happen if I mix them 50/50 and throw that in there? Be like a 77-1/2w115? Or would it be like half 80w90 and half 75w140? Would they be like forever at war with eachother over turf in my diff.? Just want to make sure here.

 

[sgtsandman]

I went with 75W140 since it’s in the manual. If you can get synthetic in the 80W90, I wouldn’t have any heartburn over it. Synthetic tends to hold up better, leaves less residue and varnish, and flows better compared to dino oil when it’s cold. That being said, there is nothing wrong with sticking with the original spec gear oil. It has worked well for a very long time.


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[Mike Tonon]

I went with 75W140 since it’s in the manual. If you can get synthetic in the 80W90, I wouldn’t have any heartburn over it. Synthetic tends to hold up better, leaves less residue and varnish, and flows better compared to dino oil when it’s cold. That being said, there is nothing wrong with sticking with the original spec gear oil. It has worked well for a very long time.


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But synthetic fluids all around could equal maybe another 1 or 2 mpg, right? I mean if the Japs can pull some nutty shit off, I don’t see why we can’t. It’d make me feel a little less envious of Toyota, ya know? If I use synthetic, I probably won’t have to change it for 200,000 miles or more, right?

 

[sgtsandman]

But synthetic fluids all around could equal maybe another 1 or 2 mpg, right? I mean if the Japs can pull some nutty shit off, I don’t see why we can’t. It’d make me feel a little less envious of Toyota, ya know? If I use synthetic, I probably won’t have to change it for 200,000 miles or more, right?

I wouldn’t extend change intervals switching to synthetic. Just my two cents.

As far as a difference in mpg, son’t expect to see any real change. I’ve yet to see a difference but the internals on my vehicles are pristine using synthetic. So that makes the added cost worth it.


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[Mike Tonon]

I wouldn’t extend change intervals switching to synthetic. Just my two cents.

As far as a difference in mpg, son’t expect to see any real change. I’ve yet to see a difference but the internals on my vehicles are pristine using synthetic. So that makes the added cost worth it.


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If I remember right, my 87 Ranger owners manual said that the original diff fluid was good for 100,000 miles and if I changed it to 75w140 full synthetic, it was good for the life of the vehicle. Why would they lie?

 

[sgtsandman]

To lessen maintenance costs and down time. All manufacturers do it.

That being said, I think you mis understand me. The rear and front axles should get changed according to the manual. I am strongly suspect of the “lifetime” info on synthetics. They can last longer but not indefinitely.

The 30,000 mile comment is for the transmission only.


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[Dirtman]

Why would they lie?

Use 75w140 then... It's your money! Why would they lie for 125 years saying 80w90 works?

Most new cars say never change the trans fluid either. Why would they lie?

Oh right... Because the warranty is over at 100k and they couldn't care less what happens after that....

 

[Mike Tonon]

What would happen if I used ATF in the diff and 75w140 in the transmission?

 

[Dirtman]

Nothing. Ford recommends that... Please do it.