[TheRuss]

OK, so I've been doing lots of research on manual transmission and diff fluids over the last few days. Fluids here cost a fortune compared to in the states. It'll cost me around GBP 100.00 just to change out the transmission and diff fluids (not including the cost of buying a pump to suck them out and refill them).

So, I firstly asked my oil supplier, who seems to be prettyknowledgable - they supply all different sorts of automotive oils and offer a "recommendation" service. Firstly I asked about using the Castrol LSD oil in my open diff - this is what they came back with:

There are no advantages to running the LSD version - it won't flow as well, so the diff is likley to get hotter and there is more drag. It won't protect any better. If you want the best for it, go for the Fuchs Syn 5, Millers CRX, Red Line 75w-90 GL5, Gulf Competition or Motul Gear 300.

The oils I mention above are ester based, making them the top level of oil quality available and will give the best protection.

So it seems that the Castrol stuff isn't quote as good as this lot, and that the LSD variant isn't any better than the open diff variant, but causes more drag - I'd suspected this all along, as I can't see BMW specifying an open diff variety of oil if the LSD version from the M cars works just as well. (Also, the LSD version is cheaper than the open diff version.)

Then I moved on to asking about the gearbox and was told:

You need an MTF for the gearbox and I would go with the Redline MTL, Amsoil MTF or Fuchs Sintofluid.

So I mentioned that an ATF was often recommended for these gearboxes and told him that cold weather shifting was a concern of mine, and the reply was:

Those gearboxes are specced to use a MTF (70w-80/75w-80) as standard, but there are often cold shift issues with them. What I recommend for the best mix of protection and cold shift ease is a mix of the Redline D4 ATF and the MTL in the gearbox. Put in the whole quart of MTL and make up the difference with the D4.

I then said that I was fairly sure that an ATF was factory fill on these gearboxes and he came back with:

If BMW specify MTF1 for the gearbox, that is basically an ATF and in approximately 2000, they started to recommend MTF2, which is a normal MTF grade. I'm 90%+ sure that the spec of yours would be MTF2, making the MTL ideal on paper, but while a MTF is the correct grade according to BMW, there are some cases where it can be too thick when cold and the ATF/MTF mix helps.

At this point I was thinking that a mix of D4 and MTF would be a good bet, but thought I'd check the Redline website and see what they recommended for my 2009 N54 6MT. The website showed the D6 ATF as the fluid for my car... so I decided to email Redline and find out what they really thought (never trust these online recommendations - they're probably filled in quickly by somebody that has to fill in recommendations for thousands of cars).

Dave at Redline emailed me back with the following:

Thank you for contacting Red Line Oil, in your 135i manual transmission the D6ATF is recommended as BMW is calling for the MTF-LT-3 fluid with the N54 engine, matching closely it’s viscosity and fluid characteristics.

An ATF is suitable for use in a modern manual transmission where components are specifically designed for it’s use.

D4ATF is popular in late model BMW transmissions where the MTF-LT-1 and 2 are called for matching the viscosity, some are also using the MTL with good results. The MTL being a higher viscosity could be stiffer shifting at lower temperatures with synchros designed for an ATF.

So I asked if it was OK/advisable to do a mix of D6 and MTL, and got this reply:

If your transmission is calling for the MTF-LT-3 fluid the D6ATF would be our recommendation.


The MTL being a higher viscosity would provide better potential protection and noise reduction, satisfying the 80W viscosity used in the earlier BMW transmissions.

Which doesn't relaly answer my question as to whether I should mix them.

At this point, I have no idea what to go for. I read the article my Mike Miller regarding fluids for the manual transmissions and he recommended MTL because it would offer better lubrication and protection, and he said that ATFs make very poor lubricants, which obviously isn't good for any bearings in the transmission. From that statement, I'd be inclined to use the MTL for its lubrication, though due to its thickness I don't know if it'd ruin the shifting of the car, especially in the cold.

I'm pretty sure at this point that the Redline 75W-90 fluid will go in my diff though, as that seems to be a bit of a no-brainer, but does anybody have any advice or anything to add that would help with my choice of transmission fluid? It seems that even the "pros" that know about this stuff can't agree, so I'm left utterly confused. Also, is my gearbox likely to be MTF1, 2, or 3? I presume 3, as per Redline's statement?