Front diff oil, additives?

[ToothPick]

I hear that some gear oils have friction modifiers, and that in some applications such oils are not recommended.

I need to change the front diff oil (75w/90) in a 2011 Subaru Impreza automatic, in which the front diff is separate from the auto transmission.

If I read correct, then a transmission/front diff combo needs something that adds friction (for the tranny to grip gears better?), while a standalone diff does not.

Which oils have additives that increase friction? Which brand do I need to get? Do the bottles list friction modifiers? Should I stay away from those with such modifiers?

 

[Donald]

I would not buy a gear oil with an FM additive. Buy plain gear oil and add OEM FM in the amount specified by the manual if called for.

The Amsoil website has a wealth of information.

Not knowing your exact engine, it looks like they call for an 80W for the front diff. You may need to get that from a dealer. It only takes a few pints, so even if expensive per pint/QT, its not really expensive.

Usually a FM is added to a limited slip diff. I just changed the rear diff. gear oil in my pickup and used Amsoil AVG 75W90 and 8 oz of Mopar FM. The 8 oz of FM was more expensive than the gear oil.

 

[stchman]

With Subaru and their AWD system, I would stick to exactly what the manufacturer wants to go in each of those boxes.

Here is the Amsoil site entry for all the gearboxes and fluids that they take.

http://www.amsoil.com/mygarage/VehicleLookupPage.aspx?FromIndex=1&url2=2011+SUBARU+IMPREZA+F

 

[Johnny248]

I've never had to add FM to the Amsoil. Most gear oils have it in already from what I've read on the bottles.

 

[jake88]

Most synthetic gear oils are going to have the FM included if it is designed for auto axles (GL-5, MIL-L-2015E). Usually it will say in the marketing language if it is friction modified "to prevent noise/vibration/harshness" in locking differentials.

The FM will not be a problem in your front differential if it is stand-alone, it just will not provide any benefit. The FM works by being easily sheared off of the surface, so the attractive force is pretty weak - it won't really displace AW/EP additives from the gear surfaces.

 

[Donald]

Originally Posted By: jake88
Most synthetic gear oils are going to have the FM included if it is designed for auto axles (GL-5, MIL-L-2015E). Usually it will say in the marketing language if it is friction modified "to prevent noise/vibration/harshness" in locking differentials.

The FM will not be a problem in your front differential if it is stand-alone, it just will not provide any benefit. The FM works by being easily sheared off of the surface, so the attractive force is pretty weak - it won't really displace AW/EP additives from the gear surfaces.


Is this really true?? The Amsoil website suggests adding 8 oz of Mopar FM for pickups like mine with locking diff, and no FM for those with a plain diff.

 

[jake88]

Originally Posted By: Donald
Originally Posted By: jake88
Most synthetic gear oils are going to have the FM included if it is designed for auto axles (GL-5, MIL-L-2015E). Usually it will say in the marketing language if it is friction modified "to prevent noise/vibration/harshness" in locking differentials.

The FM will not be a problem in your front differential if it is stand-alone, it just will not provide any benefit. The FM works by being easily sheared off of the surface, so the attractive force is pretty weak - it won't really displace AW/EP additives from the gear surfaces.


Is this really true?? The Amsoil website suggests adding 8 oz of Mopar FM for pickups like mine with locking diff, and no FM for those with a plain diff.


It depends on the marketer, but some fluids will have the FM included and some will not. You just have to look at the marketing language. If it is there, they will claim it because that extra additive costs money and is there to add value. And yes, the FM will not cause any harm in a plain differential.

**Also, not sure why on Earth Amsoil would suggest Mopar FM rather than their aftermarket version. My company's aftermarket FM has data showing improved FM durability compared to OEM fluids - so there are aftermarket additives out there that are both better and cheaper than the Mopar stuff.

 

[artificialist]

The reason FM oils are not recommended in Subarus is because FM interefres with synchro engagement. Since the open differential fluid is separate from the transmission, having friction modifiers or not will make no difference. Just buy a good 75w90 gear oil and use it with confidence.