Fluid Capacities

The Fortwo and Roadster use the same gearbox casing so the fluid capacity is the same.
It is worth noting that the new smart fortwo (451) gearbox is much larger so takes more fluid.

450: 1.4 litres
451: 2.3 litres
452: 1.4 litres
454: 2.5 litres

Thanks to AC , Scott and Jordan for the capacities.

Smart 450 And Roadster Compatible Transmission Fluid MB 235.71

Available in UK

MB Own Brand Q0022037V00000000
Mobil 1 ATF D21065
BP Autran DX III
BP Autran DX III G-34083
Castrol TQ Dexron III F-30520
Castrol Transmax DexIII Multivehicle (thanks Andy)
Comma AT and PSF MVATF Plus (thanks (thanks John M)
Veedol ATF Dexron III F-30521
Veedol Unitrans M Dexron III F-30521
Comma AQ3 (thanks Paul)

Update - Obviously things change and I'm not about to even attempt to keep this list up to date.
Search Google, local motorfactors or eBay for MB 235.71

When the smart was 1st released, it was filled with Dexron II from the factory but this eventually
changed to Dexron III and now that's what they recommend you use over the previous version.

Smart 451 & 454 Compatable Transmission Fluid MB 235.72

Castrol BOT 328 (thanks Gonzalo)
Castrol Syntrans Multivehicle 75W90 (thanks Guido)
Castrol Transmax Manual Multivehicle (thanks John F)
Burma BOT 328 (thanks Blackdog)
Mercedes own brand A 001 989 51 03 (Thanks John F)

Update - Obviously things change and I'm not about to even attempt to keep this list up to date.
Search Google, local motorfactors or eBay for MB 235.72

Smart 453 Compatable Transmission Fluid

5 Speed Manual Transmission MB253.10

MB Own Brand A 001 989 84 03
Gear Oil MB 317

6 Speed Dual Clutch Transmission MB235.73

MB Own Brand A 001 989 98 03 09
Castrol Syntrans FE 75W
Castrol Transmax Manual FE 75W

The 450/Roadster Transmission Fluid Confusion

If you ask on any forum what transmission fluid should be used in a 450 or Roadster,
you will almost certainly be told ATF (Dexron III). However, this information could be wrong.

It seems strange that although the 450 gearbox is a manual the same as the 451, the
specced transmission fluid is different? I have contacted Getrag but have had no answer. 

However

Smart's own workshop instruction system says that the 6 speed transmissions found in the fortwo
and Roadster are covered by document BB00.40-P-0236-14A which states the filing capacity of
1.4 litres of ATF listed in Sheet MB standards MB 236.14

The other information that I can't ignore is that ATF is red and that all transmission fluid I have changed
in 450's and Roadsters is also red when it came out. Dirty as hell but still red. Standard gear oil (as
recommended in 451's and forfours) listed under MB 235.72 is a normal golden colour which ties in
with what I have seen after changing the transmission fluid on a pair of 451s.

Stick with ATF for the 450 as it has worked so far and it's what Getrag designed the gearbox to use.
Mainly because the 450 and 452 Roadster gearbox doesn't contain a hypoid gear.