Confused about Differential Fluid

bryan0 · Feb 18, 2007

I spent the last hour searching through old threads about which differential fluid to use. I couldn't find a straight answer to my questions.

I have a 1999 GT, and it has 75k miles. To the best of my knowledge, the fluid has never been changed, definitely not since 62k miles when i bought it. I don't know anything about if I have the traction-lock, do they all come with it? I turn my traction control button off if that means anything.

I went out and bought some non-synthetic valvoline 80w90 and some Trans-X GL-5 "limited slip gear oil additive". Is this an okay combo?

I also got the Permatex Blue RTV gasket maker, should I return it and get black or am i good with the blue?

I know I sound dumb, I've never done a diff. oil change before. :-(

the98stang · Feb 18, 2007

bryan0 said:
I spent the last hour searching through old threads about which differential fluid to use. I couldn't find a straight answer to my questions.

I have a 1999 GT, and it has 75k miles. To the best of my knowledge, the fluid has never been changed, definitely not since 62k miles when i bought it. I don't know anything about if I have the traction-lock, do they all come with it? I turn my traction control button off if that means anything.

I went out and bought some non-synthetic valvoline 80w90 and some Trans-X GL-5 "limited slip gear oil additive". Is this an okay combo?

I also got the Permatex Blue RTV gasket maker, should I return it and get black or am i good with the blue?

I know I sound dumb, I've never done a diff. oil change before. :-(

All Mustang GT's have come with Trac-loc differentials since the 1980's.

The best oil to use is a synthetic blend 70w-140. You will need 2 quarts of it. You will also need a bottle of Ford OEM friction modifier. I wish I had that part number for you, sorry.

As far as RTV, you might want to consider a factory differential gasket. RTV is a cheaper way to do the job, and might cause leaks in the future.

40th GT · Feb 18, 2007

The gear oil you bought should be fine. Except, I'd exchange the 'trans-x' additive stuff for some Motorcraft friction modifier.

Mike97gt · Feb 18, 2007

40th GT said:
The gear oil you bought should be fine. Except, I'd exchange the 'trans-x' additive stuff for some Motorcraft friction modifier.

it's XL-3 friction modifier you want. Do not use the XL-5 and I think that trans-x stuff you bought is a non ford XL-5

bryan0 · Feb 19, 2007

okay thanks guys, i'll go return the GL-5 and pick up some XL-3.

I should use 4oz. of the friction modifier, correct?
And I'd be just as good with the Blue RTV as with the black?

Mustang5L5 · Feb 20, 2007

The newer rear ends take synthetic fluid. There is even a tag on the diff that says "Synthetic fluid only"

Oh and BTW, the traction control and traction-lok are two unrelated components. They only share the word Traction between them, but nothing else

CobraRed_96_GT · Feb 20, 2007

i'd recomend 3 quarts if you can spare it, it take bout 2 and 1/3. I used Royal Purple Max-Gear synthetic 75W90 which is a perfect mach IMO. If you lived in No. Cal ill give you the rest of this bottle i have sitting here

bryan0 · Feb 20, 2007

Mustang5L5 said:
The newer rear ends take synthetic fluid. There is even a tag on the diff that says "Synthetic fluid only"

I haven't crawled under there yet to see the tag, but a 1999 would be considered "newer" so i need synthetic?

I'm going to stop by the ford dealer tomorrow to get the xl-3 and return my valvoline 80w90 and get mobil 1 75w90. Now am i good?

I appreciate all your help guys

soctty7676 · Feb 20, 2007

Yes, you will be fine with the blue. Their website actually says it's recommended for diff covers.
http://www.permatex.com/products/au...ue_Multipurpose_RTV_Silicone_Gasket_Maker.htm

Mustang5L5 · Feb 21, 2007

bryan0 said:
I haven't crawled under there yet to see the tag, but a 1999 would be considered "newer" so i need synthetic?:

I would say so yeah. It may be even earlier that they switched to synthetic in the rear

BennyBlown2v · Feb 21, 2007

CobraRed_96_GT said:
i'd recomend 3 quarts if you can spare it, it take bout 2 and 1/3. I used Royal Purple Max-Gear synthetic 75W90 which is a perfect mach IMO. If you lived in No. Cal ill give you the rest of this bottle i have sitting here

That's alot of fluid. You're only supposed to use between 1.66-1.77-liters, which is 1.75 quarts-1.87 quarts. 2-1/3 quarts is NOT correct.

Confused about Differential Fluid

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