Originally Posted by citizen_insane
I did not realize that thanks. I've just heard people say that D6 is better than D4 for colder climates, but there isn't a whole lot of info out there. I guess I don't know much about how the thickness of the lubricant will effect the longevity of the transmission.
If I remember correctly, MTF-LT1 was around 9 cSt. I've seen conflicting information about whether MTF-LT2 was ~7 cSt or ~9 cSt. MTF-LT3 is definitely a lower viscosity fluid <7 cSt.
BMW's trend is thinner, just like all car manufacturers obsessed with meeting CAFE requirements. Aftermarket fluid suppliers like Redline recommend fluids on the thinner side so they're sure that people don't complain about poor shifting with their fluids.
That being said - thinner MIGHT shift better, as long as it's still within operating range for the transmission. There are lots of other car brands out there where people complain about shifting getting worse when hot - it's too thin.
Generalizing, slightly thicker fluids will do a better job protecting bearings and the gears themselves. One might say that it's best to use a fluid "as thick as possible" which still preserves acceptable shift quality in your normal temperature range.
Furthermore, friction modifiers are added to transmission fluids which dramatically affect shift quality and how the synchros behave. Synchros need to be "grippy" and without added friction modifiers, gear/engine oils are not "grippy" and shift quality will suffer.
I prefer to select a fluid which is designed from the get-go as a manual transmission fluid, not an ATF which happens to work OK. This is why I usually stick with Redline MTL or Amsoil MTF - designed for MTs from the start. ATF does have friction modifiers too, but the design parameters don't necessarily match MTs.
The thinner fluids in the 7 cSt range are in the same range as ATF. D6 ATF is thinner than D4 ATF (because the Dexron VI spec is thinner) and may be a better match for MTF-LT3 (if that's important to you). I care more about longevity and shift quality than precisely matching a particular target viscosity.
Both RL MTL and Amsoil MTF are ~9cSt fluids which are a little thicker (better bearing and gear protection) and match the characteristics of MTF-LT1 reasonably closely.
Redline MT90 is substantially thicker and matches a 75W90 gear oil in terms of viscosity. I know people in warmer climates running MT90 in BMWs with great results too but the average Joe may run into issues. If you DIY, it's about $15 a pop to try all the gear oils you want. If you don't DIY it's far more annoying and costly to play around.
As with most of the fluids in the car - it's more important to change it regularly than to obsess about exactly which fluid you're using.