8.8 Axle Gear Oil and Modifier Question


Active Member
May 6, 2019
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!

  • Sponsors(?)


Mustang Master
Aug 18, 2017
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.


Active Member
Apr 16, 2015
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.


will be trying this sex one when I can find it
Mar 5, 2019
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:


Slap me as well as point and laugh
May 17, 2018
Enterprise AL
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?
  • Like
Reactions: 2Blue2

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
polk county florida
Well, seems pretty straight forward right?
Or is it left?
Let us back up a few feet.
Wait, which direction are we going?
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.


Active Member
May 6, 2019
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.


My mission is to out curmudgeon you
15 Year Member
Jun 14, 2004
Acworth, GA
That cover has me wondering. For that price you can get an actual axle girdle.



i'm familiar with penetration
Mod Dude
Feb 18, 2001
I’ve typically filled my rear with Mobil 1 75w-140 and did not add friction mod.
Last edited:


Advanced Member
May 15, 2018
“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
Thread starter Similar threads Forum Replies Date
Y How much gear oil do i need for an axle swap? Fox 5.0 Mustang Tech 5
E Drivetrain Rear axle gears Fox Engine Swaparoo 5
5 Rear end gear noise 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 3
5 3.55 gears for a 87 mustang automatic transmission 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 13
C For Sale Moser 8.8 Rearend Drivetrain Parts 0
A Limited Slip Rear End Grinding and Whining 1996 - 2004 SN95 Mustang -General/Talk- 1
K Rear End/differential SN95 V6 Mustang Tech 6
Alpine615 Drivetrain Driver's Side Axle Shaft Not Seating Properly In Diff 1974 - 1978 Mustang II Talk & Tech 6
5 Clunking Noise After Axle Swap Fox 5.0 Mustang Tech 0
J Expired Fox Body 1986-1993 8.8" Rear Axle Assembly 3.55 Gears - $190 Drivetrain Parts 1
J Expired Fox Body 1986-1993 8.8" Rear Axle Assembly 3.55 Gears - $190 Drivetrain Parts 0
SC_Kona_Blue Expired Moser Wavetrack And Axles With 3.73 Gear For 8.8 Drivetrain Parts 0
Smitty15us Expired For Sale New In Boxes A Superior Axles Gears Pos Unit 8.8 Housing Drivetrain Parts 1
FoxMustangLvr Complete axle w/ 3:55 gears, questions Fox 5.0 Mustang Tech 18
88-378 Rear Gears, axles, brakes, etc.. Drivetrain Parts 0
chgovfk Rear Axle gear help needed 2005 - 2014 S-197 Mustang -General/Talk- 10
riceslayer302 quick speedo gear/axle ratio question Fox 5.0 Mustang Tech 11
ShortThrow50 axles are loose in the spider gears Fox 5.0 Mustang Tech 2
B axle gearing question Classic Mustang Specific Tech 11
Orange... c-clip/axle/gear questions help?!? SN95 4.6L Mustang Tech 8
hognutz Rear diff, axles, gears etc SN95 4.6L Mustang Tech 1
flames201 need help with axles and gears on 04 gt SN95 4.6L Mustang Tech 5
H Best Stock Rear Axle/Gears? Swap axle? 1994 - 1995 Specific Tech 4
astronut1885 Got my new axle with gears, have questions. Fox 5.0 Mustang Tech 9
ThinBlue502 Fox Rebuild Axle with New or Craigslist Axle Swap? 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 20
DudeStang For Sale FS: Roush Axle Backs Exhaust Parts 2
S abs tone ring install new axle SN95 4.6L Mustang Tech 0
R Difference Rear axles Mustang II 1974 - 1978 Mustang II Talk & Tech 7
F Drivetrain Rear axle difficult to slide in Fox 5.0 Mustang Tech 4
6 Budget 1969 mustang rear end build 1965 - 1973 Classic Mustangs -General/Talk- 1
DudeStang For Sale FS: "Mystery Muffler" - Axle Back Exhaust Exhaust Parts 0
Foxslider SOLD WTB: 87-93 stock foxbody axle out of 8.8. Drivetrain Parts 0
revhead347 Drivetrain 33 spline SN axle with ABS and parking brake build 1994 - 1995 Specific Tech 2
A Finally rear axle being attended too 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 37
Mr.T. Suspension Worn out rear axle Anti wrap shock SN95 4.6L Mustang Tech 0
A Rear axle seal removal and installation 1965 - 1973 Classic Mustangs -General/Talk- 2
fredfifty Wheels-Tires drilling out axles for arp ext studs Fox 5.0 Mustang Tech 3
Foxslider Foxbody junkyard direct replacement axles 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 41
J Suspension Rear axles Fox 5.0 Mustang Tech 1
DudeStang “Mystery Mufflers” vs. Roush Axle Backs - Exhaust Comparison 2005 - 2014 S-197 Mustang -General/Talk- 7
C For Sale Moser 8.8 Complete Rear End Drivetrain Parts 0
Willybill32 Need help with choosing a good rear axle ratio 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 35
S Rear Axle and Stock Fan/Shroud help w/ 351w 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 0
Mycelus WTB/Trade Roush-axle back for 2014 GT Exhaust Parts 0
H Drivetrain Axle movement after installing 5-lug conversion Fox 5.0 Mustang Tech 2
D WTB/Trade WTB Fox body axles 4 lug 28 spline Drivetrain Parts 0
G Drivetrain Rear axle play/rattle Fox 5.0 Mustang Tech 3
Similar threads