Rear Diff Fluid
Rear Diff Fluid
Senior User
Joined: Jan 2002
Posts: 317
Likes: 0
From: Minnesota
It may be synthetic from the factory. I think it is a 85W140 or something. Check your owners manual and the tag on the differential. If it is synthetic, it may be best to use OEM motorcraft. Otherwise I like synthetic for rear differentials like Mobil 1 or Valvoline.
Senior User
Joined: Feb 2003
Posts: 447
Likes: 0
Originally Posted by db3
Spend the money on Amsoil. Its only a couple of dollars more, and you will most likely not have to replace it again.
Postmaster
Joined: Dec 2003
Posts: 2,659
Likes: 2
From: S/C Texas
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.
Senior User
Joined: Feb 2003
Posts: 447
Likes: 0
I remember when I had the back of my truck in water up to the taillights. At the time I was unaware that the vent tube had come off. I decided to check the fluid a few days later. There was a discussing milk shake in there. It took 2 changes to get all the water out.
New User
Joined: May 2004
Posts: 21
Likes: 0
it's 75-140 synthetic. That's what ford calls for. I would go to the dealership and get the factory Motocraft fluid. Ford says if you pull or tow or are in submerged in water chnage every 3,000. regular driving 100,000miles.
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Cargo Master
Joined: Aug 2002
Posts: 2,013
Likes: 6
From: Cajun Country
Ford wants $18.95 a quart for the replacement fluid.I went back with RedLine @ $7.95 a quart.Even Wal-Mart sells there brand for $8.00 a quart. 
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Post Fiend
Joined: Jun 2001
Posts: 5,228
Likes: 5
From: Running Springs CA
Mobil One in that grade is $15-16 a quart! I normally dont like to mail order something like oil, but Redline is as low as $7, plus shipping. If I submerged it frequently, I would not bother with synthetic anyway.
Jim
Jim
Cargo Master
Joined: Aug 2002
Posts: 2,013
Likes: 6
From: Cajun Country
I didnt have to mail order my RedLine fluid.I went to there website,entered my zip code and they sent me to a dealer in my area. 
I was in Discount auto parts Saturday and saw there new stock of RedLine products.Oils,lubes,grease,etc.
I was in Discount auto parts Saturday and saw there new stock of RedLine products.Oils,lubes,grease,etc.
Logistics Pro
Joined: Jul 2002
Posts: 4,821
Likes: 19
From: South Dakota
I really dont understand all the hype of putting in a synthetic in your rear diff, maybe some one can enlighten me.
I can see the reason for using a syn to top off the factory fill....but why shell out mega bucks for a syn gear oil when a conventional gear oil will do the same job and last just as long, for a 1/4 of the cost?
75-140 synthetic is the factory fill.
I personally use 85-140 Valvoline conventional and 2 4oz tubes of LS additive in my 9.75" equipped F-150.
Its been my experince that any thing lighter, like 80-90, will leak out the rear end at operation temps.
I can see the reason for using a syn to top off the factory fill....but why shell out mega bucks for a syn gear oil when a conventional gear oil will do the same job and last just as long, for a 1/4 of the cost?
75-140 synthetic is the factory fill.
I personally use 85-140 Valvoline conventional and 2 4oz tubes of LS additive in my 9.75" equipped F-150.
Its been my experince that any thing lighter, like 80-90, will leak out the rear end at operation temps.
Post Fiend
Joined: Jun 2001
Posts: 5,228
Likes: 5
From: Running Springs CA
Hypoid gears are terribly hard on oil and are one of the biggest areas of frictional loss in the driveline. So the mpg gain helps the CAFE and since synthetic base oils are not sheared down by gears, the stuff will hold up for 100,000 miles, as long as you dont get water in it. LS additive is also usually included, so you can subtract that from the cost difference.
Jim
Jim
Senior User
Joined: Apr 2004
Posts: 139
Likes: 0
Stick to "Factory"
Originally Posted by BlueRanger94
Does anyone know why Ford went from 80W90 to 85W140?
cobrah