Rear Diff Fluid
FTE Leadership Emeritus
Joined: Sep 2000
Posts: 7,298
Likes: 45
From: Southern California
Originally Posted by BlueRanger94
Ok, but why 75W140?
Posting Guru
Joined: Jul 2001
Posts: 1,914
Likes: 1
Ford recommends that your Limited Slip unit fluid be changed every 30,000 miles due to the friction modifier that is used. It absorbs moisture from the air and will condense water in the axle housing. Open or standard axles do not have this requirement, only the LS.
Logistics Pro
Joined: Jul 2002
Posts: 4,821
Likes: 19
From: South Dakota
I just happened to look at my owners manual yesterday.
It states that the front 8.8" open diff on my truck takes 75w90 dino juice, but the rear 9.75'' open diff is supposed to take 75w140 synthetic!
How is the dino juice good enough for one axel, but not the other? (i realize the that the front diff doesnt run all the time, but the shafts in side the diff do.)
Too me this seems very contradicting.
It states that the front 8.8" open diff on my truck takes 75w90 dino juice, but the rear 9.75'' open diff is supposed to take 75w140 synthetic!
How is the dino juice good enough for one axel, but not the other? (i realize the that the front diff doesnt run all the time, but the shafts in side the diff do.)
Too me this seems very contradicting.
Senior User
Joined: Apr 2004
Posts: 139
Likes: 0
hummmm...
Ya know you could probably run most any type of fluid (without any noticable differance) in the front and back...for awhile.
But Ford has Tech's that specialize in the area of "gear contact" "pivot points" friction and wear. Research has demanded specs from manufactures to be within very close tolerances when it comes to a protecting gear lube.
I can assure you it's millions of dollars worth of business for gear lube manufactures. They all compete for the recomendations from Ford, Chevy and all car producers.
Ford has a engineering dept. that runs front and back ends with multiable- gear lubes at very high speeds and under adverse conditions. The objective is to try an destroy the gears.
A S.P.C.chart is maintained on each lube used, then compaired to many manfactures. This chart will indicate what lube is best to use... to Ford these charts are the bible!
Stick with the book, and do some research for yourself on lube specs. you will be amazed as to how complex a lube. can be.
cobrah
But Ford has Tech's that specialize in the area of "gear contact" "pivot points" friction and wear. Research has demanded specs from manufactures to be within very close tolerances when it comes to a protecting gear lube.
I can assure you it's millions of dollars worth of business for gear lube manufactures. They all compete for the recomendations from Ford, Chevy and all car producers.
Ford has a engineering dept. that runs front and back ends with multiable- gear lubes at very high speeds and under adverse conditions. The objective is to try an destroy the gears.
A S.P.C.chart is maintained on each lube used, then compaired to many manfactures. This chart will indicate what lube is best to use... to Ford these charts are the bible!
Stick with the book, and do some research for yourself on lube specs. you will be amazed as to how complex a lube. can be.
cobrah
Logistics Pro
Joined: Jul 2002
Posts: 4,821
Likes: 19
From: South Dakota
Come on, an oil revolution did not happen over night.
You make it sound as if my rear end will implode because I use a dino 85w140.
As an engineer, I can assure you that dino gear oils can and do work as well as thier syn counterparts for lubrication in a gear box. A 85-140 dino lubricates the EXACT same way a 85-140 syn....Fords tests are geared for specific weights of oils, not wether its syn or dino.
Most of the wear in a rear end doesnt come from a oil wearing out....it acutally comes from the fine metal dust that is suspended in the oil, this is why you change out your diff fluid.
Do you really think that the tolerances in my rear end are tighter than my front differential, thus needing a syn gear lube as you have suggested? .
Just cause Ford recomends the most expensive gear lube, it in no way, shape, or form, means that its the only one capable for the job.
Not one person has given solid evidence on why a syn is needed in a rearend, just the popular syn is better lines.
You make it sound as if my rear end will implode because I use a dino 85w140.
As an engineer, I can assure you that dino gear oils can and do work as well as thier syn counterparts for lubrication in a gear box. A 85-140 dino lubricates the EXACT same way a 85-140 syn....Fords tests are geared for specific weights of oils, not wether its syn or dino.
Most of the wear in a rear end doesnt come from a oil wearing out....it acutally comes from the fine metal dust that is suspended in the oil, this is why you change out your diff fluid.
Do you really think that the tolerances in my rear end are tighter than my front differential, thus needing a syn gear lube as you have suggested? .
Just cause Ford recomends the most expensive gear lube, it in no way, shape, or form, means that its the only one capable for the job.
Not one person has given solid evidence on why a syn is needed in a rearend, just the popular syn is better lines.
Posting Guru
Joined: Nov 2003
Posts: 1,071
Likes: 1
From: Concord, New Hampshire
Well, id hate to but in but im gonna anyways, I personaly dont run synthetic in my truck, however, I do in my dirtbike, I am new to the syn. line, but, the lubrication atvandages seem to be much higher with the synthetic, for instance, my bike is 2 stroke, and I run a premix of 50:1 thats with synthetic, when I was running "DINO" I had to run minumum of 32:1 thats as lean as i could go, and if youve ever seen a bike running 32:1, well you actually cant see it, so from what ive been told and read from my spectro dealer, and also the amsoil dealer in town, the synthetic is supposed to lubricate better and stand up to heat, and shear, better, but im interested to hear what other people think, so i plan on following this thread, then maby ill have to change my truck over.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
Senior User
Joined: Apr 2004
Posts: 139
Likes: 0
Originally Posted by superrangerman2002
Come on, an oil revolution did not happen over night.
You make it sound as if my rear end will implode because I use a dino 85w140.
As an engineer, I can assure you that dino gear oils can and do work as well as thier syn counterparts for lubrication in a gear box. A 85-140 dino lubricates the EXACT same way a 85-140 syn....Fords tests are geared for specific weights of oils, not wether its syn or dino.
Most of the wear in a rear end doesnt come from a oil wearing out....it acutally comes from the fine metal dust that is suspended in the oil, this is why you change out your diff fluid.
front differential, thus needing a syn gear lube as you have suDo you really think that the tolerances in my rear end are tighter than my ggested? .
Just cause Ford recomends the most expensive gear lube, it in no way, shape, or form, means that its the only one capable for the job.
Not one person has given solid evidence on why a syn is needed in a rearend, just the popular syn is better lines.
You make it sound as if my rear end will implode because I use a dino 85w140.
As an engineer, I can assure you that dino gear oils can and do work as well as thier syn counterparts for lubrication in a gear box. A 85-140 dino lubricates the EXACT same way a 85-140 syn....Fords tests are geared for specific weights of oils, not wether its syn or dino.
Most of the wear in a rear end doesnt come from a oil wearing out....it acutally comes from the fine metal dust that is suspended in the oil, this is why you change out your diff fluid.
front differential, thus needing a syn gear lube as you have suDo you really think that the tolerances in my rear end are tighter than my ggested? .
Just cause Ford recomends the most expensive gear lube, it in no way, shape, or form, means that its the only one capable for the job.
Not one person has given solid evidence on why a syn is needed in a rearend, just the popular syn is better lines.
My arguement is ...
Statistical Process Control (SPC) has proven the better grades, as I have said " you could use any gear lube" maybe for the duration of your ownership of your vehicle and may never experience a rear end explosion.
As far as tollerances front and back, and are they really a need for two types of gear lube?
Under lab. conditions "you betcha"!!! So why not under normal driving conditions? You will then have the satisfaction of knowing you are using the best lube...whats a few bucks more for the better?
I will admit your point is well taken but, as a secondary alternative only.
cobrah (Use da book on this one)
Senior User
Joined: Apr 2004
Posts: 221
Likes: 0
From: East haven ct
may have something to do with the way that the diff is disconnected don't know how just a guess. Syn might be too slippery for the clutch or what ever is in there.Syn also has a much higher burn temp.
Postmaster
Joined: Dec 2003
Posts: 2,659
Likes: 2
From: S/C Texas
Originally Posted by peppy
Every 100k miles. Only if I could treat my truck right. I'm lucky to go 6 months. A reminder to change the fluid each time you submerg your differential, so the engineers say. If you stay high and dry, yea......stuff lasts about as long as anything else in or on your truck.
Again, thats why I use namebrand, but not topend stuff.
Postmaster
Joined: Dec 2003
Posts: 2,659
Likes: 2
From: S/C Texas
Yea, water almost in the doors, and at a good speed its all thrown up everywhere....but its so much fun.
My buddy actually got his truck thru water over the hood. No way I was gonna put my truck thru that much water.
My buddy actually got his truck thru water over the hood. No way I was gonna put my truck thru that much water.
Thread
Thread Starter
Forum
Replies
Last Post
generalee31184
1999 - 2003 7.3L Power Stroke Diesel
21
Jan 10, 2016 06:08 AM
VaFordman26
1973 - 1979 F-100 & Larger F-Series Trucks
3
Sep 9, 2013 10:27 PM
