Fully Synth Diff Fluid
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Cargo Master
Joined: Oct 2008
Posts: 3,330
Likes: 13
From: Eaton, CO
Fully Synth Diff Fluid
My rear limited slip is a bit jittery and I've heard changing the fluid to a fully synth 75W-140 w/ the slip additive is the way to go. The stuff isn't exactly readily available, but I've been able to find Mobil 1 LS 75W-140 which claims not to need the slip additive.
Do any of you run the Mobil 1 LS 75W-140? It's about $20/quart. Did you still add that slip additive, or was it good to go w/o the additive? I wanted to go with Schaeffers, but they only sell it in a 5gal pail or a 55gal drum. No other vehicle I have takes 75W-140 gear oil, so that's a no go for sure.
About much fluid does the rear and front diff take?
Do any of you run the Mobil 1 LS 75W-140? It's about $20/quart. Did you still add that slip additive, or was it good to go w/o the additive? I wanted to go with Schaeffers, but they only sell it in a 5gal pail or a 55gal drum. No other vehicle I have takes 75W-140 gear oil, so that's a no go for sure.
About much fluid does the rear and front diff take?
Elder User
Joined: Mar 2003
Posts: 699
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From: northern Michigan
Mine came with the full synthetic in the rear and Dino in the front. Synthetic I used required the additive for the limited slip diff. Took about three quarts. look over the cover closely as Ford has a problem with these rusting through.
Postmaster
Joined: Nov 2006
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Here's a great write up for the Rear: Welcome to guzzle's Rear Differential Maintenance Web Page
And here's a great write up for the front: Welcome to guzzle's Front Differential Maintenance Web Page
And here's a great write up for the front: Welcome to guzzle's Front Differential Maintenance Web Page
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Cargo Master
Joined: Oct 2008
Posts: 3,330
Likes: 13
From: Eaton, CO
I've read Guzzle's write-ups for the axles. I can't seem to find anything, but Valvoline LS and Mobil 1 LS 75W-140 gear oil. Everything has the LS already in it and claims to meet Ford's specs.
Post Fiend
Joined: Feb 2007
Posts: 14,541
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From: Dallas-Ft. Worth
I have to do mine here shortly. But when I do, I'm getting the non-additive stuff because I want to put in my own. Too much friction modifier makes your LS diff an open diff. I think too many folks think it's the other way. I'm going to add an ounce at a time until it JUST stops chattering and that's it. May take 2 or all 4, but if I don't need all 4, I'm not adding it.
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Cargo Master
Joined: Oct 2008
Posts: 3,330
Likes: 13
From: Eaton, CO
Mine doesn't seem to be chattering, but it feels like it's locking up upon startup while going around corners causing vibes throughout the truck.
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Hook it, I'll pull it.
Joined: May 2007
Posts: 15,037
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From: Grand Lake, La.
I have to do mine here shortly. But when I do, I'm getting the non-additive stuff because I want to put in my own. Too much friction modifier makes your LS diff an open diff. I think too many folks think it's the other way. I'm going to add an ounce at a time until it JUST stops chattering and that's it. May take 2 or all 4, but if I don't need all 4, I'm not adding it.
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Lead Driver
Joined: Jun 2003
Posts: 8,298
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From: Damon (South East Texas)
I can spin one all the way to OD. 
Don't know why Ford would put a pegleg rear axle in a Lariat 4x4. 
I need to find a good locking rear differential to keep my back tires even. 
Don't know why Ford would put a pegleg rear axle in a Lariat 4x4. I need to find a good locking rear differential to keep my back tires even.
Post Fiend
Joined: Feb 2007
Posts: 14,541
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From: Dallas-Ft. Worth
Fleet Mechanic
Joined: Dec 2007
Posts: 1,766
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From: Hutchinson, KS
Ford now says that the synthetic 75-140 can go in both the front and the rear. I checked with ford before I did mine. Even though the book calls for different fluids front and rear, you can run the exact same. I recently replaced mine with the Valvoline 75W-140 front and rear. It says that it already has the LS additive in it, but several parts/service people have said that I may still need to add some. I think they may be right cause I still get a small amout of vibration/chatter when turning tightly on dry pavement. I figure I'll do 1oz at a time until I'm satisfied.
BTW Joe- Rick's dually has a Dana 80 and it takes more fluid than yours. So just cause he used 6oz of additive doesn't really mean much for you cause the rear ends are different.
Rick, what kind of fluid did you use?
BTW Joe- Rick's dually has a Dana 80 and it takes more fluid than yours. So just cause he used 6oz of additive doesn't really mean much for you cause the rear ends are different.
Rick, what kind of fluid did you use?
Hook it, I'll pull it.
Joined: May 2007
Posts: 15,037
Likes: 4
From: Grand Lake, La.
Ford now says that the synthetic 75-140 can go in both the front and the rear. I checked with ford before I did mine. Even though the book calls for different fluids front and rear, you can run the exact same. I recently replaced mine with the Valvoline 75W-140 front and rear. It says that it already has the LS additive in it, but several parts/service people have said that I may still need to add some. I think they may be right cause I still get a small amout of vibration/chatter when turning tightly on dry pavement. I figure I'll do 1oz at a time until I'm satisfied.
BTW Joe- Rick's dually has a Dana 80 and it takes more fluid than yours. So just cause he used 6oz of additive doesn't really mean much for you cause the rear ends are different.
Rick, what kind of fluid did you use?
BTW Joe- Rick's dually has a Dana 80 and it takes more fluid than yours. So just cause he used 6oz of additive doesn't really mean much for you cause the rear ends are different.
Rick, what kind of fluid did you use?
Post Fiend
Joined: Feb 2007
Posts: 14,541
Likes: 2
From: Dallas-Ft. Worth
Thanks though -- someone else reading this may not have realized you have a bigger rear end.
Hook it, I'll pull it.
Joined: May 2007
Posts: 15,037
Likes: 4
From: Grand Lake, La.