DIY: Differential Fluid Change

Driftlo, I have recently picked up a 2016 M2... I noticed you recommended the 75w140 Castrol Syntrax in alot of the M3 variants. Given mines also a clutch style lsd (im guessing) would the castrol syntrax 75w140 be optimal when I do a fluid change in the rear diff?

Was also considering the Motul 75/140 but got lost in the details of FM. Guess that would just be up to driver preference with noise levels?

Saintjah wrote

Driftlo, I have recently picked up a 2016 M2... I noticed you recommended the 75w140 Castrol Syntrax in alot of the M3 variants. Given mines also a clutch style lsd (im guessing) would the castrol syntrax 75w140 be optimal when I do a fluid change in the rear diff?

Was also considering the Motul 75/140 but got lost in the details of FM. Guess that would just be up to driver preference with noise levels?

Friction modifier is there to change the transition between locked and open. If it's a clutch-type LSD, you need FM. The question of "how much" is up for debate. I defer to the OEMs since they tune the car to handle a certain way.

If you think you know the car well enough to adjust it to your tastes, then buy a fluid with no FM and add it in small doses until you get the behavior you want. This is going to be a time-consuming process.


Here's a discussion on it from two years ago that I started on Bob Is The Oil Guy:
https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3998486/The_argument_for/against_FM_in

This post by 440Magnum says it best:

I wouldn't say that it "reduces locking." It makes the jump between the static (holding) and static (slipping) coefficients of friction smaller, so you don't get into a vicious grab/release/grab/release cycle that results in chatter. It shouldn't significantly reduce the actual holding capacity of the LSD clutches enough to really matter, and any loss is more than offset by a more controlled onset of locking.

If it helps anyone (might have missed if it was posted), but I purchased this allen key and then had about an inch piece cut off of it. I inserted it into the drain and fill plugs and then put a 14mm ratcheting wrench over the end that stuck out.

I couldn't ever find a 14mm hex socket that was short enough to fit in there

https://www.amazon.com/Bondhus-12284-14mm-Short-L-Wrench/dp/B002YNQP5A/ref=sr_1_3?ie=UTF8&qid=1534952667&sr=8-3&keywords=14mm+allen+key

14MM STUBBY DRAIN PLUG SOCKET 3/8 DRIVE

DavidZ wrote

14MM STUBBY DRAIN PLUG SOCKET 3/8 DRIVE

hey would ya look at that

MSteve5036 wrote

hey would ya look at that

Yep, your solution is ingenious though

For those who would like a cheap, complete set of allen keys....I bought these when performing the service and had no issues.

https://www.harborfreight.com/9-piece-38-and-12-drive-metric-hex-bit-socket-set-67880.html

Steps 7 and 8 have me a little confused. Why am I lowering then raising the car with the fill plug out?

Wouldn't I just fill as much fluid as possible (some overflows out) then put the fill plug in and be done?

shimmy23 wrote

Steps 7 and 8 have me a little confused. Why am I lowering then raising the car with the fill plug out?

Wouldn't I just fill as much fluid as possible (some overflows out) then put the fill plug in and be done?

Yeah those steps are a little misguided. You can raise up the driver's side of the car higher than the passenger side to get a little extra of the old fluid out because the drain hole isn't completely flush with the bottom of the housing, but it's unnecessary when filling back up because the max fill should just be up to the fill hole.

Thank for this post.

The 500ml Castrol MBZ spec bottles work great. No oil pump needed. They fit right up in there and you can walk away while they drain.

The word on the street is you need to be doing it at a very level ground. I wonder if anyone has done it on a slightly tilted floor and how the results are?

Just did the change for the second time. On the first time I used a pump and had all 4 corners up and leveled. This time I did the following.

Jack passenger side up and place stand.
Remove fill bolt followed by drain bolt.
Lowered car.
Jack driver side up and place stand.
Have a beer while it drains.
Lowered car.
Jack passenger side up and place stand.
Replace drain bolt and torque to 60nm.
Fill Diff with nearly 3 bottles of Castro's Syntrax.
Lower car.
Allow overflow to come out of the fill hole.
Have another Beer.
Jack passenger side up and place stand.
Replace fill bolt and torque to 60nm, clean off diff.
Lower car.
Have another beer.

Much easier and less clean up then my first go.

nick3753 wrote

Just did the change for the second time. On the first time I used a pump and had all 4 corners up and leveled. This time I did the following.

Jack passenger side up and place stand.
Remove fill bolt followed by drain bolt.
Lowered car.
Jack driver side up and place stand.
Have a beer while it drains.
Lowered car.
Jack passenger side up and place stand.
Replace drain bolt and torque to 60nm.
Fill Diff with nearly 3 bottles of Castro's Syntrax.
Lower car.
Allow overflow to come out of the fill hole.
Have another Beer.
Jack passenger side up and place stand.
Replace fill bolt and torque to 60nm, clean off diff.
Lower car.
Have another beer.

Much easier and less clean up then my first go.

This.

I just changed my diff fluid with this method. Drivers side lifted first will let more pour out of the drain side, passenger side up in the air will allow you to pour all 3 bottles in while lifted without pouring out. Then you drop the passenger side, let it all drain on even ground (it took about 5 minutes to stop dripping) then re-lift the car and pop in the fill plug. Done.

Only catch is on the last step my oil drain pan was too high to rest under the car to drop the car completely and let the diff fluid pour out evenly and settle, so I had to use a cardboard box instead cut in half to catch the 1/2 bottle as it settled.


I tried to think of everything, but now I need a low profile drain pan next time!

I'm planning do this soon. Everyone making it sound way easy… I'm little confused though

You can get by just jacking one side up and down ?

How often should the diff be replaced?

Thanks!

They're tipping the car right to allow for more thorough drainage. Then tipping it left to slightly overfill. Then letting it drain on level ground. Not sure what the interval is on these cars but i do every 40k on my Subarus with LSD.