Gearbox 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 236.14

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 236.14

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.

ATF Vs Gear Oil

You hear it a lot online where people say that you can use either 75w90 gear oil or ATF in a smart gearbox.

Just because you can, doesn't mean you should. Getrag who makes the gearboxes obviously know what they are talking about as they make some impressive transmissions for some amazing cars. They design the gear box to use a specific type of gear box fluid. There are no benefits to using the wrong type of fluid. Neither are rare or expensive.

Differences

ATF is a thinner fluid designed to lubricate by increased flow movement.
Gear oil is more viscous and therefore clings to the gears.
The thicker gear oil can introduce viscous drag which reduces efficiency.

ATF expands when it gets hot.
Gear oil burns off when it gets hot.
If you use gear oil, keep an eye on the levels.

ATF acts like a coolant inside the gearbox.
Gear oil is far less efficient at cooling.
Above spec temperatures aren't a massive issue but could cause issues with seals over long durations.

ATF is not a lubricant. It's a specialised fluid that also has lubricating properties.
It contains many specialised additives such as viscosity index improvers, kinematic index improvers,
rubber seal rejuvenating agents, sheer transfer additives, gasket conditioners, corrosion inhibitors, detergents etc.

Gear oil is (normally) a synthetic lubricant containing extreme pressure additives and anti wear additives.
It contains these in order to be able to run correctly in hypoid gears.

ATF is designed to run in gearboxes using straight cut, spiral bevel, helical tooth and planetary style arrangements.
Smart 450 and 452 gearboxes do have helical tooth cogs but they don't have hypoid gears.

ATF has a longer life than gear oil which would need a more regular changing frequency.
Since smart (wrongly) decided that the gearbox fluid wasn't something that needed changing,
they went for a fluid with a longer life span.