Another diff oil thread

[911Fiddler]

(Because I’m tight) I’m trying to avoid buying BM OE diff oil (it’s £150 ish for the 3 req bottles here in the UK) and I was hoping my diff would be ok on straight Castrol, but it wasn’t, loads of unwanted aggressive locking on car park turns, roundabouts, etc.

1-fresh Castrol base oil; not good.
2-added 25cc of RedLine FM; better, less intervention.
3-added another 25cc of RedLine; better still, almost gone entirely.

So I’m on 50cc, just under 5% FM. Don’t want to go too far because I’m told it’ll thin out the oil, could cause diff wear.

Views & experience on this, how much FM to add?

[Steve]

Understand being tight, I am too, but there is no playing around with the diff. Get the correct OE BMW oil and don't worry about it again for a very long time. I'd also put a new cover on the back with new bushings while you're at it.

[911Fiddler]

Understand being tight, I am too, but there is no playing around with the diff. Get the correct OE BMW oil and don't worry about it again for a very long time. I'd also put a new cover on the back with new bushings while you're at it.

Thanks. May end up going for OE oil, but not just yet. Many sound to be satisfied with the non OE mix up. Ive been told that there is a possibility that the diff friction plates are shot, by a guy who sells a repair service, but then he would say that. Thus too much investment in this one would be wrong.

[oceansize]

Thanks. May end up going for OE oil, but not just yet. Many sound to be satisfied with the non OE mix up. Ive been told that there is a possibility that the diff friction plates are shot, by a guy who sells a repair service, but then he would say that. Thus too much investment in this one would be wrong.

See this thread. At 5% you are already at the limit of what anyone recommended.

[SteelGreyM]

Diff fluid is somewhere I dont hold back on. BMW makes their magic formula for this.

[Tbonem3]

The molybdenumdisulfide FM will not thin out the oil and/or cause accel wear (actually MOS2 is an anti-wear additive in oils), but what it will do, if too much is present, is allow the LSD to slip too much rendering it lifeless 'round turns.

My opinion is, if not just using the OE lube, is to use as little FM as possible, even putting up with some minor chatter or groaning as that's better than a peg leg.

[oceansize]

I feel OP's pain. I installed a 3.91 diff a few weeks ago and of course had to buy the diff oil, but the really good news is I have to buy the oil again after the recommended 1200 mile break-in period . I love spending money on oil.

[heinzboehmer]

I feel OP's pain. I installed a 3.91 diff a few weeks ago and of course had to buy the diff oil, but the really good news is I have to buy the oil again after the recommended 1200 mile break-in period . I love spending money on oil.

Hopefully you bought it from FCP!

[Sharocks]

I feel OP's pain. I installed a 3.91 diff a few weeks ago and of course had to buy the diff oil, but the really good news is I have to buy the oil again after the recommended 1200 mile break-in period . I love spending money on oil.

But but but, one of the best mods. Love that gearset in this car.

[Radekxpl]

About 8 years ago I was researching this "magic BMW potion" fluid and was able to get technical data spec of the actual OE BMW fluid. In deed the original BMW diff fluid contains exactly 4.6% of FM (Friction Modifier) exactly 69ml per 1.5L (1500ml) ususal diff fluid capacity or 23ml per 500ml bottle also friction modifier they use is the same Ford uses in their diffs and can be purchased separately as well Ford Motorcraft XL3 http://rover.ebay.com/rover/1/711-53...QAAOSwDOpb2LN6. I have been using this formula for past 8 years with excellent results as matter of fact just changed my diff fluid last weekend according to this spec 151k on it not a single problem. Works exactly like OE BMW diff fuild, change it every year since I track the car frequently. I wish I saved the spec sheet I had, can't find it anywhere now By the way thread on the old m3forum was my posting. Hope this helps

[elbert]

About 8 years ago I was researching this "magic BMW potion" fluid and was able to get technical data spec of the actual OE BMW fluid. In deed the original BMW diff fluid contains exactly 4.6% of FM (Friction Modifier) exactly 69ml per 1.5L (1500ml) ususal diff fluid capacity or 23ml per 500ml bottle also friction modifier they use is the same Ford uses in their diffs and can be purchased separately as well http://rover.ebay.com/rover/1/711-53...QAAOSwDOpb2LN6. I have been using this formula for past 8 years with excellent results as matter of fact just changed my diff fluid last weekend according to this spec 151k on it not a single problem. Works exactly like OE BMW diff fuild, change it every year since I track the car frequently. I wish I saved the spec sheet I had, can't find it anywhere now By the way thread on the old m3forum was my posting. Hope this helps

Just in case someone needs this in the future (ebay listings expire eventually), he linked "Ford Motorcraft XL3" friction modifier

.

[Johnvu]

About 8 years ago I was researching this "magic BMW potion" fluid and was able to get technical data spec of the actual OE BMW fluid. In deed the original BMW diff fluid contains exactly 4.6% of FM (Friction Modifier) exactly 69ml per 1.5L (1500ml) ususal diff fluid capacity or 23ml per 500ml bottle also friction modifier they use is the same Ford uses in their diffs and can be purchased separately as well http://rover.ebay.com/rover/1/711-53...QAAOSwDOpb2LN6. I have been using this formula for past 8 years with excellent results as matter of fact just changed my diff fluid last weekend according to this spec 151k on it not a single problem. Works exactly like OE BMW diff fuild, change it every year since I track the car frequently. I wish I saved the spec sheet I had, can't find it anywhere now By the way thread on the old m3forum was my posting. Hope this helps

This is good to know! I will try this out next time I have to change the fluid or switch up the gearing. Thanks!

[911Fiddler]

About 8 years ago I was researching this "magic BMW potion" fluid and was able to get technical data spec of the actual OE BMW fluid. In deed the original BMW diff fluid contains exactly 4.6% of FM (Friction Modifier) exactly 69ml per 1.5L (1500ml) ususal diff fluid capacity or 23ml per 500ml bottle also friction modifier they use is the same Ford uses in their diffs and can be purchased separately as well http://rover.ebay.com/rover/1/711-53...QAAOSwDOpb2LN6. I have been using this formula for past 8 years with excellent results as matter of fact just changed my diff fluid last weekend according to this spec 151k on it not a single problem. Works exactly like OE BMW diff fuild, change it every year since I track the car frequently. I wish I saved the spec sheet I had, can't find it anywhere now By the way thread on the old m3forum was my posting. Hope this helps

Excellent info thanks.
But to throw a fly into the soup, I’m told there are different types of FM additive, ie differing chemical approaches. I’d not mentioned my first go at this, I’d used some FM by a company called Westway; this stuff made the diff groan very badly. Their tech dept mentioned that the base Castrol oil already has some FM in it, unknown %, unknown type.

[Radekxpl]

Just in case someone needs this in the future (ebay listings expire eventually), he linked "Ford Motorcraft XL3" friction modifier

.

Good Point 👍 Yes that is exactly it Ford Motorcraft XL3

[Radekxpl]

Excellent info thanks.
But to throw a fly into the soup, I’m told there are different types of FM additive, ie differing chemical approaches. I’d not mentioned my first go at this, I’d used some FM by a company called Westway; this stuff made the diff groan very badly. Their tech dept mentioned that the base Castrol oil already has some FM in it, unknown %, unknown type.

Yes this is definitely true about FM additives just like Oils not everyone is created equal.