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Curious George
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500 Posts
Discussion Starter · #1 ·
I ordered my rear IRS from Mike Forte https://cobradreams.com/2019/12/11/ordered-the-irs-center-and-knuckles-today/ and it had to be drained since it wasn't on a pallet...

I know many of you have installed the IRS, and I don't want to start a holy war - but I've seen tons of info on Amsoil, Royal Purple, Redline and Motorcraft oils

One thing is the Royal Purple 75W-90 gives you friction modifier in the mix already which means no mixing needed...


Amsoil 75W-90 has the new squeeze pouch version - gimmicky? or useful?


Redline now makes a 75W-85 with friction modifier


or for those tried and true folks, Motorcraft XY-75W-85-QL


with Ford Racing Friction Modifier


what did you use? and why? :coffee:
 

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FFCobra Craftsman
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25,055 Posts
For the 8 yrs or so I ran a solid axle w/ Ford lim slip I always ran Mobil 1 which is rated for an LS diff. I don't worry about the modifier. My understanding is that it is used to make the LS work smoothly in average driving. I believe it does that by making it a little less limited and a little more slip. I want my LS tight so i have never run the modifier. I did a 2015 IRS retrofit 3 yrs ago and run Mobil 1 in it also. Works fine to me.
 
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I used the Ford products only because it was recommended. What ever you use shop around for the best prices. I got mine from my local ford dealer and felt like I could have gotten it much cheaper.
JR
 
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For the stock Ford Mustang 2015+ IRS, I use the exact products Ford recommends. No reason IMO to reinvent the wheel or introduce any risk. Also from a local Ford dealer.
 

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Depends on the differential you have. I have a Detroit TrueTrac in my IRS and here is what the Eaton Performance Differential Product Owner's manual recommends: Detroit Truetrac - High quality mineral gear lubes are required for use in Detroit Truetrac differentials. Regardless of the lube type, always use a GL5 rated lube with the least amount of friction modifier. Mineral lubes lacking friction modifiers (limited-slip additives) were historically recommended for all Truetrac applications because friction modifiers can slightly reduce the bias ratio (limited-slip aggressiveness) of Truetrac differentials.
 
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Curious George
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500 Posts
Discussion Starter · #6 ·
for the 2015+ IRS the Ford recommended is this: SAE 75W85 Synthetic Specs

Rear axle fluid: Motorcraft SAE 75W-85 Premium Synthetic Hypoid Gear Lubricant XY-75W85-QL WSS-M2C942-A
Rear axle friction modifier (U.S. and Mexico):1 Motorcraft Additive Friction Modifier XL-3 EST-M2C118-A

Note: For a complete drain and refill, limited-slip axles require addition of Friction Modifier, Part Number XL-3 meeting Ford specification EST-M2C118-A.
Refer to Workshop Manual or Owner’s Guide.
For a complete drain and refill, Torque Vectoring RDU requires addition of Torque Vectoring RDU Gear Oil Additive, Number XL-19

I'm unsure what the Torque Vectoring RDU would be - I'm thinking it's an AWD component...
 

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Premium Member
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9,134 Posts
Hi Todd,
Any of the oils you have listed will be fine. Earlier model Mustangs called for 75W-140 to reduce bearing and gear failure. The IRS calls for 80W90 now. Either will work fine. I'll use 75W-140 in my car because I plan on track use...
Thanks again for your business... Get that car driving soon!
 

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Junior Charter Member
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2,915 Posts
I've never used a squeeze pouch, but I bet it would work well. For the bottles, there is a pump that you can get for about $5 that screws on the top. I have also used a syringe for a top up.
 
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