Gear Oils with Friction Additives - Are OEM additives still needed?

Joined
Nov 8, 2022
Messages
4
Location
SoCal
New member here.

My truck:
2003 Ford F-350, XLT, 4X2; 6.8L; 4R100
80K miles
Towing travel trailer and boats (3K-8K) several times per year...distances between 50 miles and 2000 miles

Preparing for a rear differential service. My rear axle tags are as follows; it is a Ford axle:
10.5"; 3.73, Limited Slip
75W140 Synthetic Oil Only


Most gear oils claim to have "friction modifiers for limited slip applications" (ex. Amsoil, and Motorcraft).
Do these gear oils contain the sufficient amount required and is it of the appropriate type for the Ford 10.5" LS axle?
Should the Motorcraft XL-3 Addtitive Friction modifier be added, regardless of whether the oil contains it or not?
My manual states to use 8oz. of Motorcraft XL-3 additive (two 4oz bottles).
Is too much additive going to cause adverse reactions, considering most oils have it in them already?

What gear oils are you using?
I use Motorcraft in engine oil and Motorcraft in trans fluid.
I read several threads/posts (here and other Ford forums) where it appears most are not using Motorcraft, but are using....Amsoil, Mobil 1, Red Line, Royal Purple, and a few others.
Any reason Motorcraft is not too popular?

Thanks!
 

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
23,073
Location
Iowegia - USA
@SoCalAngler Warm it up and drive around in tight figure 8's and see if you can hear any screeching noises from the differential or feel any grabbing sensations.

If not you don't need any extra differential friction modifier.

If you do hear noises or feel grabbing, add only 1/4 of a tube at a time.

Just to add to the understanding of Friction Modifiers:

Each fluid type and fluid application has a different additive package and contains different Friction Modifier and AW chemistry for the specific materials used in those transmissions or differentials.

The friction modifiers in engine oils are NOT the same friction modifiers as found in ATF's. The friction modifiers in ATF's are NOT the same friction modifiers as found in LSD gear oil.

For an LS differential with flat plate or cone clutches that friction modifier is used to prevent chatter.

In MTF's, the friction modifier is used to enhance synchro engagement/disengagement. The two chemistries are entirely different and react differently.

In engine oils the Friction Modifiers are primarily there to reduce friction.
 
Last edited:
Joined
Dec 19, 2019
Messages
3,985
Location
Muncie, Indiana
Any reason Motorcraft is not too popular?
Because for the most part with Motorcraft fluids you get an okay fluid at a price level that you could get something better, like from an engine oil perspective, Synblend Motorcraft costs more than full synthetic Quaker State at Walmart and I don't think anyone would argue that the Motorcraft is better.
 
Joined
Jan 22, 2011
Messages
7,870
Use a gear oil that meets the specs for your differential. I am not brand loyal. If you use a gear oil meant for limited slip differentials, there is no need to add a friction modifier. I have read too much friction modifier is not a good thing either and can affect the smooth operation of the clutches in the differential.
 
Joined
Dec 26, 2005
Messages
23,636
Location
Upper Midwest
Because for the most part with Motorcraft fluids you get an okay fluid at a price level that you could get something better, like from an engine oil perspective, Synblend Motorcraft costs more than full synthetic Quaker State at Walmart and I don't think anyone would argue that the Motorcraft is better.
Plus sometimes there is a lot of imagination on the Internet.
 
Joined
Jul 18, 2008
Messages
101
Location
Gaul
My manual states to use 8oz. of Motorcraft XL-3 additive (two 4oz bottles).
Is too much additive going to cause adverse reactions, considering most oils have it in them already?

In a clutch-type LSD ("Positraction"), slip additive reduces your LSD action, making it more and more like an open differential the more slip you add. It's a balance between how much bias/grip you want and how much noise you are willing to put up with.

Typically, manufacturers overdo the slip because people want absolutely no noise or chattering. Typically, GL5 oils with slip additive already in them contain plenty for the same reason.

If you want to experiment, start with an oil without any additive, like Redline's 75W140NS. Probably it will result in so much grip the truck is nearly undriveable. Tame it down by adding slip until you're happy. Redline also sells slip additive separately. Add it 1oz or less at a time, then drive around the block to check.

Seems to me if you are regularly pulling boats up wet launches, you want good LSD action so don't go overboard.
 
Joined
Dec 6, 2019
Messages
933
Location
South
No additives needed. Use the Motocraft synthetic. I changed my 2003 Ford at 145,000 and it looked as good as the new fluid that went in.
 
Joined
Jul 12, 2012
Messages
7,302
Location
Caldwell Idaho
The wife used to have a 2002 ford F250 with the 7.3 Powerstoke 6 speed manual and a posi rear end ,the differential oil would come out looking like there was a problem and the uoas would show lots of metals and solids. I did a couple changes and started to use extra friction modifier and that cured them probem. I used M1 75W-140
 
Joined
Oct 26, 2008
Messages
1,310
Location
Northern, NY
My '99 F350 with the 10.5" Sterling required 1 bottle of the Ford friction modifier to work smoothly with Amsoil 75w-140. Without a load in the bed it was fine, but with a load you could feel the differential pop/chatter on turns. I started with 1/2 a bottle, but that didn't totally eliminate the chatter - the second half of the bottle was just enough.

Our 2016 Ford Expedition required 2 bottles of the Ford fiction modifier to eliminate the chatter with RP 75w-90.
 

SoCalAngler

Thread starter
Joined
Nov 8, 2022
Messages
4
Location
SoCal
My '99 F350 with the 10.5" Sterling required 1 bottle of the Ford friction modifier to work smoothly with Amsoil 75w-140. Without a load in the bed it was fine, but with a load you could feel the differential pop/chatter on turns. I started with 1/2 a bottle, but that didn't totally eliminate the chatter - the second half of the bottle was just enough.

Our 2016 Ford Expedition required 2 bottles of the Ford fiction modifier to eliminate the chatter with RP 75w-90.
I have the same axle and my owner's manual states 8oz. of Motorcraft XL-3 friction modifier. It comes in 4oz bottles. Are you referring to adding one 4oz. bottle? Sounds like since the Amsoil 75W-140 already contains some modifier, 1/2 the required amount was enough.
 

SoCalAngler

Thread starter
Joined
Nov 8, 2022
Messages
4
Location
SoCal
A couple of follow up questions...

1. If you start with NO additional friction modifier, and some is needed after test driving, do you suck out, ex, 4oz. of gear lube, and then add 4oz of friction modifier?

2. Not wanting to start a brand war thread.....Is Amsoil 75W-140 as acceptable as M1 or other brands?

Thanks!
 

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
23,073
Location
Iowegia - USA
A couple of follow up questions...

1. If you start with NO additional friction modifier, and some is needed after test driving, do you suck out, ex, 4oz. of gear lube, and then add 4oz of friction modifier?

2. Not wanting to start a brand war thread.....Is Amsoil 75W-140 as acceptable as M1 or other brands?

Thanks!
That all depends. If the oil level is below the fill plug, where it should be, then just add the FM. If it runs when you open the drain plug then reduce it and add your FM.

Check the bottle label or the product data sheet and see if it says LS anywhere on it. If it does, it contains some FM.
 
Joined
Oct 26, 2008
Messages
1,310
Location
Northern, NY
I have the same axle and my owner's manual states 8oz. of Motorcraft XL-3 friction modifier. It comes in 4oz bottles. Are you referring to adding one 4oz. bottle? Sounds like since the Amsoil 75W-140 already contains some modifier, 1/2 the required amount was enough.
Yes, one 4oz bottle.
 

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
23,073
Location
Iowegia - USA
Here is a caution: Do not overtreat.

If you overtreat beyond what is just needed, then you will have slipping and you won't get proper lockup when it is really needed.
 

SoCalAngler

Thread starter
Joined
Nov 8, 2022
Messages
4
Location
SoCal
I will start with Amsoil Severe Gear oil 75W-140 that has LS additive in it. I will not add any additional. I'll test in figure 8 pattern and see if any noises are heard. If noises/chatter is heard/felt, I'll add one half the OEM recommended dose (OEM rec. 8oz.) which is 4 ounces.

Thanks all!
 
Top