There is no service schedule on changing the smart gearbox oil. For higher milage vehicles, it is wise to take the precaution and change the oil. Other reasons to change the oil may include a replacement gearbox, or any process which will require the drive shafts to be removed.

The replacement gearbox oil is listed as AQM (Dexron II) for all smart fortwo's and Roadsters.
The capacity of the gearbox is 1.4 litres.

The Drain plug is located at the bottom of the bellhousing.
With the car at normal operating temperature, use a 1/4" Hex drive to remove the plug, drain the fluid in a suitable container.

Once drained, tighten the drain plug up (35Nm).
There is a sealing compound available from smart if you want extra sealant, though this is not required. Part number SQ000 0282 V000.
Now undo the fill hole plug using the same hex drive.

Pour the new fluid in. The fill level is until the fluid is level with the bottom of the fill hole.

Now tighten (to 35Nm) the filler screw up using the compound above if necessary.

Remember to dispose of the old oil correctly!