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2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Differential Oil Change
I have 56,728 miles on my 2015 SL400 and decided to change out the rear differential oil. There’s no scheduled maintenance for doing that unless you have a limited slip differential. Mine is non-limited slip. To my surprise, the stuff that drained out looked like coffee (without cream and sugar). The stuff I put in was Castrol Syntrax Long Life 75W-90 and looked like golden honey.
Has anyone else change their rear differential oil and was it black?
Last edited by JettaRed; Jun 21, 2025 at 10:22 PM.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Thanks everyone for the replies. I went ahead and changed out the diff oil in my C350, as well. I also had a fine shimmer in the oil in both cars, though the C350 with 134,803 miles did not appear to be as dark (wish I had taken a picture of that, as well).
I happened to have had the Castrol Syntrax in my garage. I had bought a couple of liters a few years ago and never got around to using it. It's hard to get any gear oil that is not GL5 or made for limited slip differentials. This fluid is specifically for non limited slip diffs. I'm not sure Castrol offers it under that name anymore. When looking at my past purchases on Amazon, it is now called Castrol Transmax Axle. The insane thing is that I paid $15.99 per quart in March 2022 and it is now over four times as much. WTH, over! Mine was produced in Germany. The new stuff just says manufactured in the EU. Interesting!
Do you think the fact that this meets some BMW specification will hurt my Mercedes?
Last edited by JettaRed; Jun 22, 2025 at 06:15 AM.
I believe "non slip diff" chemistry is tolerant of brass parts.
I could not find anything except compatible with "non-slip"
Diff oil needs to be "hypoid gear" rated not simply gearbox duty.
When I recently did the rear diff in my 2013 E 63 with 73,000 miles, the fluid did come out rather dark. I assume it was the first time it had ever been done in the vehicle which I had picked up with 42,000 miles. I do believe and could be wrong, but the fluid used in my rear diff was green (fuchs?) to start so the fact that it was rather dark didn’t bother me. That’s when I discovered my rear leaking axle seal. I refilled it with some Valvoline Synthetic GL five and now the dealer will put in whatever they stock. No limited slip for me.
Theres some overlap with 75w90 transmission and gear oils, it depends on marketing but they can both carry the same ILSAC GL-4 GL-5 rating. There will always be metal in the oil, it's really not a huge deal, most of it comes out in the first 5000 miles and it sorta just stays in there. Everybodys use-case is different. My car barely sees hills at high temp, but I'm running the 75w90 severe gear which is a 100% synthetic formulation. I don't feel a difference but at least now I don't have to think about it.
Then you get into the oddball stuff like we put in Porsches like Mobil Delvac and Swepco due to their specific formulation and anti wear additives which are good for transaxles. An open diff car isn't exerting any serious pressure nor does it need a specific lubricity for use with LSD's so you can pretty much use any ol' 75w90.
I had replaced my rear diff fluid a few years ago prior to finding out about all the Mercedes specs. At that time I had bought and used Syntrax long life 75W-90. Shortly after changing the rear, within a week, I found out about the MB specs. I then found a Febi brand fluid that has the proper MB spec printed on the bottle so replaced it again. I only had the Syntrax fluid in about a week but the car drove fine.
I forget where the Syntrax idea came from, I read it somewhere that it was good to use. I had to track it down and it wasnt cheap either.
When I did replace the rear diff fluid the first time I do remember thinking how good the old fluid looks, like it must have been recently replaced and I didnt even need to do it. I have no idea when or if that fluid was replaced prior, could have been replaced right before I bought the car. I forget exactly how the front diff fluid looked but I dont remember thinking it looked as good as the rear looked and glad I did it.
Didnt want to waste the Syntrax so I use it in the auger gearbox of my 1971 Ariens snowblower.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Originally Posted by ChrisHimself
Theres some overlap with 75w90 transmission and gear oils, it depends on marketing but they can both carry the same ILSAC GL-4 GL-5 rating. There will always be metal in the oil, it's really not a huge deal, most of it comes out in the first 5000 miles and it sorta just stays in there. Everybodys use-case is different. My car barely sees hills at high temp, but I'm running the 75w90 severe gear which is a 100% synthetic formulation. I don't feel a difference but at least now I don't have to think about it.
Then you get into the oddball stuff like we put in Porsches like Mobil Delvac and Swepco due to their specific formulation and anti wear additives which are good for transaxles. An open diff car isn't exerting any serious pressure nor does it need a specific lubricity for use with LSD's so you can pretty much use any ol' 75w90.
Thanks, Chris. I have some Mobil 1 Delvac 75W-90, but can't remember why I got it.
We get to look at bad pinion bearings and shaft bearings totally matured...
That shows that good oil chemistry and service are essential to limit glittery self-destruction.
1x Qt of oil goes along way to 50kMi Max!
There is no pump in there to circulate oil so lube chemistry has to be effective against frictions. OEM oil limits results with mandatory service intervals.
> TIPS ARE...
PAO hypoid gear oil
neodymium magnetic plug
timely service interval
don't repair parts, replace units!
Last edited by CaliBenzDriver; Jun 24, 2025 at 12:46 AM.
Is 75w90 the recommended weight? I also have a 2025 SL400. Capacity is 1L? Is my diff nonslip?
I think its "235" spec lets you use dang near anything 75-90, 85-90, and 90 SAE was the last time I looked. The important part is that it's clean and synthetic so it handles the heat and pressure in the scenario that you decide to do 155mph uphill in the thing for 30 minutes straight. Mercedes specifications involve severe-use case scenarios like can your brakes stop from max speed in freezing weather, and can your oil not break down in the UAE on your G-Wagen driving in 115F weather, and will your air filter survive catching on fire.
Generally yes, 75-90 and 85-90 are the accepted viscosities for Mercedes passenger car differentials. The limited slip differentials are typically AMG cars and even then typically they would need to specifically be optioned with them in the past so if you had an SL55 or SL63 you'd have to check your VIN to see what your car was optioned with. Those use a specific Castrol Syntrax 75-140 with friction modifiers for 235.61, now superceded by Castrol TransMax 75-140. Again I'm leaving room for me to be incorrect here but thats what I'm told to order for said vehicles.
Liquimoly does not have the MB Approval for said items, but they will often have the matching BMW approval and the API GL5 standard which the Transmax/Syntrax also has so common sense says youre probably ok. Again a lot of these companies just buy Castrol base oil and blend it anyway. Amsoil does it with Mobil/Castrol according to my knowledge.
I think its "235" spec lets you use dang near anything 75-90, 85-90, and 90 SAE was the last time I looked. The important part is that it's clean and synthetic so it handles the heat and pressure in the scenario that you decide to do 155mph uphill in the thing for 30 minutes straight. Mercedes specifications involve severe-use case scenarios like can your brakes stop from max speed in freezing weather, and can your oil not break down in the UAE on your G-Wagen driving in 115F weather, and will your air filter survive catching on fire.
Generally yes, 75-90 and 85-90 are the accepted viscosities for Mercedes passenger car differentials. The limited slip differentials are typically AMG cars and even then typically they would need to specifically be optioned with them in the past so if you had an SL55 or SL63 you'd have to check your VIN to see what your car was optioned with. Those use a specific Castrol Syntrax 75-140 with friction modifiers for 235.61, now superceded by Castrol TransMax 75-140. Again I'm leaving room for me to be incorrect here but thats what I'm told to order for said vehicles.
Liquimoly does not have the MB Approval for said items, but they will often have the matching BMW approval and the API GL5 standard which the Transmax/Syntrax also has so common sense says youre probably ok. Again a lot of these companies just buy Castrol base oil and blend it anyway. Amsoil does it with Mobil/Castrol according to my knowledge.
Can confirm the Transmax for the AMG spec - The dealer just did a diff fluid change on my 2022 AMG GLE and used the Transmax - here is the bottle they used:
Theres an online store IDparts https://www.idparts.com/febi-oil-hyp...5-p-15664.html i'll buy from. They deal with diesel vehicles but have the same various parts. Bought most of my transmission service parts here. They are located the next town over in MA and have free pick-up at there warehouse.
When I bought the Febi oil it was around $18/bottle but has gone up since. they have Fuchs also.
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