Differential fluid

[N8Dogg98]

I have a 1998 Mustang GT with 3.73's that were installed about 6 months ago. The rear cover on the differential is leaking, and to fix the problem I purchased a new cast aluminum girdle which I would like to install shortly. What viscosity fluid should I use, 75W 90, or 75W 140? Also, do I have to add a friction modifier when I change the fluid? If so, how much? I would like to use Mobil 1 synthetic gear oil because it is readily available at my local auto parts store.

 

[BennyBlown2v]

You should run a synth 75w90 will work fine. Go with royal purple, doesn't need any friction modifier added. I went with 75w140 valvoline synth and 4oz of ford friction modifier and it doesn't lockup great. Less modifier you add the more bite you'll get...but the faster everything will wear.

 

[Kilgore Trout]

What does it say in your owner's manual? Is there a tag on the diff?

Basically you should be using one of these two choices:
-full synthetic 75W-140
-conventional 80W-90 gear oil

If you have trac-loc you need to add 4 ounces of friction modifier

 

[BennyBlown2v]

What does it say in your owner's manual? Is there a tag on the diff?

Basically you should be using one of these two choices:
-full synthetic 75W-140
-conventional 80W-90 gear oil

If you have trac-loc you need to add 4 ounces of friction modifier

Bad advice. Ford switched over from 75w90 to 75w140 to improve longevity of parts (mainly "clutch packs") in a seemingly random choice one year. Plus, besides RP, Mobil 1's full synth stuff has built in friction modifier. Adding more friction modifier to a full synthetic oil which already has the proper mix of chemicals isn't going to help anything, it will just make it worse.

I think what you meant Kilgore was...if your car is daily driven, you never beat on it, and don't care if you lockup both wheels when power is applied, put whatever you want in it

 

[Kilgore Trout]

Ford switched from 80W-90 to 75W-140... Not sure where you are getting 75W-90 And it was not "one year" the switch was made and Ford did not switch back the next year.

I do not care what Royal Purple has in it, we are talking about a diff no need for snake oil in the diff You must add friction modifier to the Ford lubricant, why would you buy any other lubricant but Motorcraft?

Like I said look in your owner's manual and look for a tag on the diff. If you are thinking about putting RP in there, just give me the money instead

 

[BennyBlown2v]

Ford switched from 80W-90 to 75W-140... Not sure where you are getting 75W-90 And it was not "one year" the switch was made and Ford did not switch back the next year.

I do not care what Royal Purple has in it, we are talking about a diff no need for snake oil in the diff You must add friction modifier to the Ford lubricant, why would you buy any other lubricant but Motorcraft?

Like I said look in your owner's manual and look for a tag on the diff. If you are thinking about putting RP in there, just give me the money instead

I was getting 75w90 from the fact that is what most synthetics come in. As for Ford they switched it only in the mustangs, but didn't switch it in their trucks/suv's and w/not even though it's the same exact diff. RP is not a snake oil product, their diff fluid is really good, and the LSD modifier they use is perfect for use in trac-lok diffs. I've used conventional oil, RP, and full synth + friction mod in both of the diffs I've had in my car, RP works best in this instance... However, Royal Purple engine oil is just regular oil, same with their ATF fluid, etc... Hell, even the gear oil is just "regular oil"...it's just that the fluid is premixed with a quality modifier, in the exact right proportion for a conventional friction disc-type limited slip diff. Oh yeah, and if you've ever priced full synth 75w140 you would know that RP isn't exactly on the expensive end - hell ONE bottle of Mobil 1 stuff is $16-$18 at PepBoys, Autozone, O'Reily, etc.

 

[N8Dogg98]

I was wondering if I need a different fluid since the gears were changed from 3.27 to 3.73. I don't have a manual and the tag is gone on the diff. It sounds like synthetic 75W 140 and 4oz friction modifier will work...unless someone is certain that the Mobil 1 fluid has the friction modifier already in it. Thanks for the input, I appreciate it.

 

[Kilgore Trout]

You can download your owner's manual in PDF format from www.ford.com

It is difficult to find and you have to register but it is there.

 

[mity2]

royal purple i used. good stuff. i didn't add additive, but seems to be working fine as i can leave 2 marks

 

[myponyrocks]

I was wondering if I need a different fluid since the gears were changed from 3.27 to 3.73. I don't have a manual and the tag is gone on the diff. It sounds like synthetic 75W 140 and 4oz friction modifier will work...unless someone is certain that the Mobil 1 fluid has the friction modifier already in it. Thanks for the input, I appreciate it.

To the best of my knowledge there is no reason to change the recomended fluid due to a gear swap.