If your car is an older smart fortwo or roadster, and you have some oil usage problems, it might not be a bad idea to check for leaks, one of the more common leakage points is the dipstick tube seal. You cannot buy this seal by itself, it comes as part of the dipstick tube assembly. I will show you how to change your dipstick tube, it's a 10 minute job, and can be done with all panels still on the car.

You will need:
E10 socket
Ratchet
Small socket extension bar.
Dipstick assembly from smart (approx £18).
Kitchen towel

Notes
There are three different assemblies for the dipstick tube, one for 600cc smarts, one for 700cc smarts, and one for diesels, give smart your chassis number and they'll order the right one for you.

600cc part number - SQ0003060V0040000000
Diesel dipstick is SQ0001428V003000000
700cc petrol one is SQ0012847V002000000

or if you're feeling frugal, you can follow the same procedure, but just change the o-ring

O ring for dipstick (Q0002470V000000000) Green O ring £1.03 (thanks to Cabbage from Smartmaniacs)

Procedure

Step 1:
Get your car ready to work on, remove the engine lid, and keep to one side.

Step 2:
Remove and wipe down the dipstick from the tube guide. This is to ensure that when you remove the tube, there's no oil inside held in by the vacuum of the dipstick seal, the same effect as if you have your thumb over the end of a straw, the liquid stays in the straw, so take the dipstick out or you'll get oil all over your nice shoes.

Step 3 (E10 socket):
Using the extension bar, and socket, undo the top E10 torx bolt, keep it to one side.

Step 4 (E10 socket):
Remove the extension bar, and undo the bottom E10 torx bolt.

Step 5:
Withdraw the dipstick tube through the top of the engine, you'll be left with this.

Step 6:
Feed the new dipstick tube down through the top of the engine and let it rest near the hole.

Step 7:
Get under the car again, wipe off any oil around the face of the dipstick tube opening, and feed the tube into it's hole in the engine block.

Step 8 (E10 socket):
Bolt up the bottom bracket until it's just nipped up.

Step 9 (E10 socket):
Using the extension bar again, bolt up the top dipstick tube bracket back to the engine, nip up tight, and do the same for the bottom bracket, it should be fairly tight so the seal fits well.

Step 10:
Check your oil level using your nice new dipstick.

Afterthoughts
This is a closeup of the seal that perishes, mine was in a rather bad state and you can see the oil runs around the lower bracket.

You can also see in some of the pictures the sheer extent of the oil around the engine bay. The oil hits the driveshaft at speed and spatters it all over your engine.

Usual disclaimer applies, if you break your car when you do this procedure, it's your fault.