I Really Don't Know Why But...

There are some normal, intelligent smart owners out there who insist that you can use gear oil in transmissions that are designed and specified to be used with ATF. Apparently the manufacturer of the gear box don't know what's best.

Their arguments rely heavily on 2 things.

The gear box isn't actually an automatic.
They have used gear oil for years and it's been fine.

However

The actual physical gear box is very robust so you could probably run one of these gear boxes for years on olive oil.
Of course you wouldn't though because it's not what it's designed for.

Also, the design of the gearbox cogs and the way it distributes the fluid decides on what fluid it uses.

ATF & Gear Oil Are Very Different

ATF has a much lower viscosity than gear oil, ATF viscosity is close to 5W/10 oil.
Gear oil is much thicker with a standard cold rating of 75W. It reduces power output through viscous drag.

Gear oil is an oil (hence the name).
ATF isn't an oil. It's a hydraulic fluid with added lubricants.

ATF doesn't foam up due to anti-foaming additives and a lower viscosity.
Gear oil foams like a rabid dog.

ATF has detergents to keep gear box and galleries clean (no galleries in smart gearbox).
Gear oil doesn't clean the gear box.

ATF acts like a coolant.
Gear oil is designed to handle heat, friction and high pressure.

ATF contains a lot of anti-wear and anti-corrosion additives, see below.
Gear oil contains 1 additive.

ATF and Gear oil is designed to work with different synchro cone clutch lining materials.
Using the wrong fluid can cause unnecessary long term wear to your synchros.

Gear oil is designed for spiral beval gears (also known as hypoid gears). These gears have a sliding action
requiring an oil suitable for high pressures. ATF gear boxes do not have spiral beval gears.

ATF Additives

Anti-wear, anti rust and corrosion, dispersants and surfactants, kinematic viscosity and viscosity index improvers,
anti-foaming, anti-oxidation, cold flow improvers, high temperature thickeners, seal swell & gasket conditioners.

Gear Oil Additives

Sulphur based anti-wear.

Viscosity

Unlike an engine, the transmission fluid is not pumped. It is splashed and flung by the submerged gears.
Thinner ATF can be splashed and flung much further and easier than the thicker gear oil.



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.

MB Standard

Dexron II

From the factory, the 450 fortwo and Roadster comes with ATF Dexron II. However, if you are replacing the fluid, I would recommend using ATF Dexron III. It's a superior update to the previous standard and 100% backwards compatible with transmissions that use the previous standards. Dexron III is easier to buy, more common and usually cheaper.

What brand should I buy?

It doesn't really matter. The internals of the 450/452 gearbox are very robust and Dexron III
specifications are fairly strict. Just buy what you can find. If you have a prefered brand, use it.

ATF Sounds Great, Can I Use It instead Of Gear Oil?

No. If your transmission requires gear oil, it was designed for gear oil.

Fluid Or Oil?

All oils are fluid but not all fluids are oils.

What If I Ignore You?

Initially nothing but eventually you'll need a new gearbox.
Seems like a lot of money just because you continued to use the wrong fluid.