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[Deszus]

75W-90 oil, you need 2 liters all together.
Don't go with thicker like 75W-140. Will lost fuel efficiency.

I refilled with Motul 300 75W-90. That sh#tty oil Merc recommends is crap. Go with Motul. MUCH smoother gearbox. Much easier gears changing. I can compare results only to what I had before - diluted black oil with steel particles in it. What can I say. Only better it is every way. Worth every penny.

I am not Motul seller. I like quality products. Castrol is far from quality oil. Check web for opinions.

[BlackCDI]

Not a big motul support in Germany. Liqui moly however...

[Deszus]

So go with Liqui Moly. Make sure any 75W-90 oil you use is API-GL4 compatible.

"API Category GL-4 designates the type of service characteristic of spiral-bevel and hypoid gears in automotive axles operated under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications.

API Category GL-5 designates the type of service characteristic of gears, particularly hypoids in automotive axles under high-speed and/or low-speed, high-torque conditions. Lubricants qualified under U.S. Military specification MIL-L-2105D (formerly MIL-L-2015C), MIL-PRF-2105E and SAE J2360 satisfy the requirements of the API GL-5 service designation."

Motul 300 75W-90 is API-GL4 compatible and API-GL5. Try to find similar from Liqui Moly.

[mr singh]

That sh#tty oil Merc recommends is crap.

in what way is the stuff mercedes sell crap?

iv used it twice and had no issues with it!

[Brabs]

Can’t agree that Castrol oils are crap. That statement is based on nothing! They are a world leading brand of lubricants sponsoring amongst others the Renault and McLaren F1 teams. They have an over 100 year history of producing lubricants.   

[CrazyG]

..... I can compare results only to what I had before - diluted black oil with steel particles in it....

And whose oil was it that you had in your gearbox ?
Who is to say that the previous owner (or garage that serviced it used
the best/proper oil for the car ? It could have been any old stuff they
had lying around.)
 
So I agree with the comments of mr singh and Brabs.
And I believe in facts...not the 'opinions' of unqualified fools on the Internet.

[Deszus]

That sh#tty oil Merc recommends is crap.

in what way is the stuff mercedes sell crap?

iv used it twice and had no issues with it!

You can use vegetable oil in gearbox and have no issues with it. My problem with Merc is that they only approve one oil from one manufacturer for 454 gearbox? Hahaha, seriously? Can't stop laughing. Any MB-Approval on oil is only grade agreement between companies.

API and SAE standards are something more.

I wonder what MB approval code has vegetable oil from Tesco.

[Brabs]

That sh#tty oil Merc recommends is crap.

in what way is the stuff mercedes sell crap?

iv used it twice and had no issues with it!

You can use vegetable oil in gearbox and have no issues with it. My problem with Merc is that they only approve one oil from one manufacturer for 454 gearbox? Hahaha, seriously? Can't stop laughing. Any MB-Approval on oil is only grade agreement between companies.

API and SAE standards are something more.

I wonder what MB approval code has vegetable oil from Tesco.

Really? Based on what information exactly?! 

[Alex]

We're getting off track. Sure you can pour anything in the gearbox (engine or fuel tank for that matter), but you'll ruin it immediately.

GL4 oils contain additives that when squeezed between the synchros they form a sacrificial layer to prevent wear of the soft metals.

That's why you can't use strictly GL5 oils in ours, that has extreme pressure additives that actually wear out our synchros.

I think we all appreciate your excitement for your car, we all love forfours, just try to understand that we've been taking care of ours for quite a while now and without the proper maintenance they wouldn't be on the road anymore.

I think you had the misfortune of getting a tired example and that's why you're so adamant that factory stuff is crap, but look around, there's only ~12year old cars here, do you really think that if the factory fill was bad there'd still be any of them around? How come no manual gearboxes failed until now? Even if the oil wan't great, considering it has so little effect, why bother?Stick a magnet on the gearbox and invest the time and effort in what really matters in a car, like cool wheels or bigger speakers.

[CrazyG]

I hate to say it, but I agree 100% with everything that Alex has just said.

So please, let us keep to the original topic/thread, ok.

[Deszus]

That's why you can't use strictly GL5 oils in ours, that has extreme pressure additives that actually wear out our synchros.

I will explain that for oil manufacturers. API standards GL4 and GL5 are in interest here.

GL4 don't attack non-ferrous metals: brass, bronze, aluminum, copper
GL5 high pressure additives will slightly oxidize non-ferrours metals: brass, bronze, aluminum, copper

TRUE

But...
If manufacturer says, that oil is compatible with GL4/GL5 it means, that this kind of oil contains metal deactivators to protect brass synchros and aluminum case. Wants more? How and why? Start studying inorganic chemistry like I do for years on my own. I don't have plans to drive ForFour to next 10 years. I am engineer and knows there is math behind everything. Only wants things to be clear about gearbox oil in ForFours.

Let's start dance.

Why those gearboxes still works? Manufacturer did good job. Does it make sense to keep same oil for hundred thousands miles and many years? Doesn't for me. That oil cooked so many times change it's own properties. After 200k miles not changing it, you can have there bucket of water. No difference. Why? Because additives. If you run clean mineral oil and filter/clean it off metals particles from time to time, you can run it forever. I run same CNC machine on pure mineral oil for years. It works every day on same pure mineral oil. Cuts heavily, tons of different materials daily. Machine only filters that oil. Constantly use same oil for years.
But gearbox oil contains sulphur additives which degrade thanks to heat and friction. This is reason to change that oil let's say every 50k miles.
 
New oil in gearbox provides easier gears changing, it's MUCH smoother (sound) when cruising on highways. It means less friction. It means better MPG. It's not theory. It's my experience with ForFour gearbox.

So it works for me. It's cheap and easy to change that oil.
Yea, engine oil is much more important because has more features than only lubricating like gearbox oil, but it doesn't change sense of replacing gearbox oil from time to time.