Diff fluid friction modifier mix percentage?

[macaroni86]

There was a thread I saved at M3F about differential fluid changes and in the thread there was a percentage of FM modifier that you could add to standard BMW SAF-XJ fluid that was the equivalent of the more expensive pre-mix fluid. I think it was something like 4.6% (something like 46ml per liter) but I can't find anywhere else that mentions it to confirm it. I'll be doing the change while I replace output shaft seals and CV joints so I'm trying to find this info again before draining the differential.

[Tbonem3]

No one knows. I'm sure some have gotten the "perfect" amount at some point. Try a couple of ounces of fm (molybdenum disulfide). You'd rather have too little than too much. Too little and there will still be a little groaning on tight turns. Too much and your lsd clutch will slip defeating the purpose of an lsd.

[oceansize]

A guy in this thread, who seems to have some legit credentials if you read to the bottom, says 2% FM for Castrol SAF-XJ. Take it for what it is worth.

Edit: That would be roughly 1oz of FM since 1.5 litres ~ 50 ounces.

[BigRussia]

When I did my diff service I used the OEM Castrol (75w140) differential fluid kit from ECStuning, along w/ a bottle of Redline FM (its a small bottle, was $12 at the time). Improved and quieted down my tired high mileage diff big time, was a very noticeable difference! (probably didn't help the that the old fluid was totally black and disgusting)

[macaroni86]

Thanks for the help. Too little to start is probably sage advice, 2% sounds like a good starting point. It might have even been 2.6%, who knows! Can always add more, can't take it out.

[JamesSJ1]

I recall the original thread and followed it when I did my change at 60k mi. I used ~2/3 of the 114ml (4oz) Redline FM bottle, which would put you around 75ml. FCP seems to indicate that the latest version no longer needs the FM. I did the change at 90k and didn't add FM (figured I could add it later as needed), and am only about 1000 miles into it in a few months. Other than slow, sharp turns in a parking lot, I never notice the diff grab any differently before or after the recent drain/fill.

Edit: someone posted this on the review for Syntrax on FCP Euro:
Note this review is for those with a BMW M3 E9x. The differential oil does not have the friction modifier (FM) like the BMW OEM differential oil. You will see a bunch of information by reading some of the various M3 forums. Long story short, the FM in the BMW OEM fluid is 4.6%. What that equates to is about 23ml of FM per 500ml bottle that is needed to make this equivalent to the current OEM formula. I'm not going to go into what FM you should add as there are plenty of debates, but wanted to make sure those buying it know what they need to do to make this fluid current BMW OEM like. P.S. you will still save a ton by adding the FM yourself over buying OEM. I hope this helps those buying this product!

[RevUpp]

4.6%

https://www.m3post.com/forums/showthread.php?t=155103

[r4dr]

I recall the original thread and followed it when I did my change at 60k mi. I used ~2/3 of the 114ml (4oz) Redline FM bottle, which would put you around 75ml. FCP seems to indicate that the latest version no longer needs the FM. I did the change at 90k and didn't add FM (figured I could add it later as needed), and am only about 1000 miles into it in a few months. Other than slow, sharp turns in a parking lot, I never notice the diff grab any differently before or after the recent drain/fill.

Edit: someone posted this on the review for Syntrax on FCP Euro:
Note this review is for those with a BMW M3 E9x. The differential oil does not have the friction modifier (FM) like the BMW OEM differential oil. You will see a bunch of information by reading some of the various M3 forums. Long story short, the FM in the BMW OEM fluid is 4.6%. What that equates to is about 23ml of FM per 500ml bottle that is needed to make this equivalent to the current OEM formula. I'm not going to go into what FM you should add as there are plenty of debates, but wanted to make sure those buying it know what they need to do to make this fluid current BMW OEM like. P.S. you will still save a ton by adding the FM yourself over buying OEM. I hope this helps those buying this product!

This is my plan when I get to this. I bought 3x of the 500 mL Castrol fluid from FCP and a small bottle of FM from RockAuto (Motorcraft XL3). I plan to end up at 4.6%.

[bigjae46]

I recall the original thread and followed it when I did my change at 60k mi. I used ~2/3 of the 114ml (4oz) Redline FM bottle, which would put you around 75ml. FCP seems to indicate that the latest version no longer needs the FM. I did the change at 90k and didn't add FM (figured I could add it later as needed), and am only about 1000 miles into it in a few months. Other than slow, sharp turns in a parking lot, I never notice the diff grab any differently before or after the recent drain/fill.

Edit: someone posted this on the review for Syntrax on FCP Euro:
Note this review is for those with a BMW M3 E9x. The differential oil does not have the friction modifier (FM) like the BMW OEM differential oil. You will see a bunch of information by reading some of the various M3 forums. Long story short, the FM in the BMW OEM fluid is 4.6%. What that equates to is about 23ml of FM per 500ml bottle that is needed to make this equivalent to the current OEM formula. I'm not going to go into what FM you should add as there are plenty of debates, but wanted to make sure those buying it know what they need to do to make this fluid current BMW OEM like. P.S. you will still save a ton by adding the FM yourself over buying OEM. I hope this helps those buying this product!

For the E9X M3, I'm pretty sure if comes with XJ from the factory. If a customer has a low speed groaning complaint, then it will get XJ+FM. I'm sure there's a TSB or test plan out there for this. XJ has a lower amount of FM in it. Or BMW might have started putting XJ+FM in from the factory at some point.

Lots of E9X M3 customers were complaining about the groaning noise early on.

[r4dr]

For the E9X M3, I'm pretty sure if comes with XJ from the factory. If a customer has a low speed groaning complaint, then it will get XJ+FM. I'm sure there's a TSB or test plan out there for this. XJ has a lower amount of FM in it. Or BMW might have started putting XJ+FM in from the factory at some point.

Lots of E9X M3 customers were complaining about the groaning noise early on.

I read a few threads on M3Post recently – they say the E9X M3 came with SAF-XJ early on, with SAF-XJ+FM specced for customers who complained about noise. But at some point they forwent the regular SAF-XJ and just put XJ+FM in everything regardless of noise complaints or not.

Probably keeps it simpler + heads off groaning complaints resulting in a second visit and twice the labor + parts.

[SandeepM3]

e46 m3 came with SAF-XJ 75w-140 originally as well. Service bulletin for e46 also says if customer complains add FM. If customer comes back again, add more. Then they eventually just specced it in the oil becasue they dont want to have to pay for warranty work. If you are ok with a little clutch chatter dont even use it.

[bigjae46]

You want some amount of FM so the lockup is smoother. Any gear oil that says it is compatible with a limited slip diff has FM in it.


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[Casa de Mesa]

What does the diff chatter sound like? Is it a groan? Or is it more of an actual chatter where it sounds like it gearing that locks then releases rapidly?

I had my diff completely drained for probably 5 months earlier this year while redoing the entire rear suspension. I replaced all the diff seals. Put Castrol SAF-XJ back in it, but did NOT put in friction modifier. I've previously run this setup (no FM) on the last two flushes prior to this one and have had zero noises. Now, however, I'm getting a sound that is similar to a Detroit locker.

Full disclosure... I also switched to solid aluminum diff bushings (all 3 - two in the diff cover, one in the subframe/front diff bushing). Save me the flame, I will likely switch back to BME OE due to NVH.

Given the above, I'm wondering if what I'm hearing is 1) the bushings transmitting noises, or 2) diff that was pretty dry after 5 months of no fluid and may need some FM, or 3) a combo of both.

[DanAvoN7]

What does the diff chatter sound like? Is it a groan? Or is it more of an actual chatter where it sounds like it gearing that locks then releases rapidly?

I had my diff completely drained for probably 5 months earlier this year while redoing the entire rear suspension. I replaced all the diff seals. Put Castrol SAF-XJ back in it, but did NOT put in friction modifier. I've previously run this setup (no FM) on the last two flushes prior to this one and have had zero noises. Now, however, I'm getting a sound that is similar to a Detroit locker.

Full disclosure... I also switched to solid aluminum diff bushings (all 3 - two in the diff cover, one in the subframe/front diff bushing). Save me the flame, I will likely switch back to BME OE due to NVH.

Given the above, I'm wondering if what I'm hearing is 1) the bushings transmitting noises, or 2) diff that was pretty dry after 5 months of no fluid and may need some FM, or 3) a combo of both.

When making tighter turns it will sound like a bit of a groan and you will physically feel it locking/unlocking as the car is moving. It's a slip stick effect. What likely happened was there was some residual friction modifier left on one of the previous fluid changes (some in the case and some worked in to the clutches) and now after the flush with the regular SAF-XJ fluid (no FM) you are experiencing the locking.

Any new noises you are hearing while driving perfectly straight are due to the solid aluminum differential mounts. You are likely hearing some pretty loud drivetrain noise (gear whine) due to these mounts. For anything short of a dedicated track car I recommend sticking to a new set of stock Genuine OE BMW mounts.

[bigjae46]

There’s a lot of residual gear oil left after a simple drain. I’d say you get an easy .5l if you remove the rear cover and let it drain overnight.


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