8.8 Axle Gear Oil and Modifier Question

Red50Fox

Active Member
May 6, 2019
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Afternoon Folks. I'm almost done "dry fitting" all of my parts and start locking things down on my 5 lug disc brake upgrade (88 GT Vert) and I've read through a whole bunch of info regarding filling up the Rear Axle.
Here is what I have:
  • 8.8 with new 3.73's and a new diff cover (https://www.americanmuscle.com/frpp-finned-88-diff-cover.html) which is a bit roomier than the stock metal can the Axle came with.
  • 2 qt bottles of Royal Purple MaxGear 75 - 90, included with the gear kit.
  • 1 4 oz. bottle of Ford Performance Friction Modifier as well as 1 4 oz. bottle of Motorcraft Friction Modifier (1 of these must have come as part of a kit).
When it comes to filling the differential I generally hear "fill it up till it runs out the hole". My plan is to use 1 bottle of the friction mod first to ensure I get a full 4 ounces in then fill the remainder till it "runs out the hole"

Here is my question: With the larger diff cover (more capacity) should I put some or all of the 2nd friction modifier in and then fill it with the 2 quarts of 75 - 90 oil? There appears to be such a thing as too much friction mod. Should I just use the 1 bottle of friction mod and get another bottle of RP 75-90 and top it off?

I would really like to not trash the axle because I did not use the right amount and type of lubricant.

Please share your years of experience with me!

Thanks
 
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Steel1

Mustang Master
Aug 18, 2017
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I would imagine the Royal Purple gear oil is synthetic, if so when I've used synthetic gear oil it had the modifier already in it.
Should specify on the label.
 

KRUISR

Active Member
Apr 16, 2015
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I used the Royal Purple (synthetic) bought at local parts store and the modifier is in it (I didn't use an additional modifier). 13,000 km later no issues.
 

2Blue2

will be trying this sex one when I can find it
Mar 5, 2019
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Isn't the friction modifier for the clutch pack of the posi?
You didn't state weather your is a trac loc or not.:thinking:
 

Olivethefet

Slap me as well as point and laugh
May 17, 2018
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Isn't the friction modifier for the clutch pack of the posi?
Thats what I've read. I'm going out on a limb here, but if his avatar is a pic of his car. If it is he has a GT Vert. Didn't all the GTs come with a trac loc rear?

Is it possible to have too much friction modifier? and if so how much would be to much?
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Well, seems pretty straight forward right?
Or is it left?
Let us back up a few feet.
Wait, which direction are we going?
Right
Left
Forward or reverse?
Clear as mud
I used the royal purple gear lube and the modifier when it got the gear change and clutch pack rebuilt. I don't cut my ride no slack,
Don't over think it, fill it to the hole and add the modifier, drive it around the block or do a few figure 8s in a parking lot, check the level then drive it like it owes you money.
 

Red50Fox

Active Member
May 6, 2019
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Don't over think it, fill it to the hole and add the modifier, drive it around the block or do a few figure 8s in a parking lot, check the level then drive it like it owes you money.
I wish I could not over think it. I'd have been done with this a while ago. But, in my own defense it's been decades since took on a project like this. Thanks for the information.
 

revhead347

My mission is to out curmudgeon you
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That cover has me wondering. For that price you can get an actual axle girdle.

Kurt
 

Mustang5L5

i'm familiar with penetration
Mod Dude
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I’ve typically filled my rear with Mobil 1 75w-140 and did not add friction mod.
 
Last edited:

96pushrod

Advanced Member
May 15, 2018
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“Recommended Upgrade. When changing your differential cover it is recommended to upgrade to fresh Royal Purple Synthetic Gear Oil. You will need 3 quarts of Gear Oil.“

This, as well as most diff covers that aren’t of the stamped steel variety use 3 quarts of fluid. I’ve never personally been a synthetic gear oil kind of guy. Lots of manufacturers recommend just using conventional gear oil in their carriers

Regardless of whether the oil has the friction modifier or not I still add a bottle of the motorcraft stuff. You’ll get different answers as to oil weight, but I run the 75w-140 stuff. :poo: takes all day to fill 3 quarts worth with how thick it is.

I’ve also ran the 80w-90 before without issue. I’d just use what you got
 
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