seicento transmission

AFAIK you can remove the hub and driveshaft and replace the transmission end boot without removing the engine. You might also consider replacing the hub end CV boot at the same time. Both are probably of the same age. Failure of this boot allows water and road grit to enter the CV joint, which is more serious than losing a bit of transmission oil. Cost depends on where you go, dealerships are expensive. The job should be well within the capability, of a well recommended, local, self employed, mechanic. Get more than one quote. The boots are not expensive, it's mostly time and labour. It's a not uncommon problem, and is really a question of how much oil, and how much it irritates you. If it's a significant amount of oil then maybe the boot has a split in the folds where it flexes, in which case it needs replacing. In passing, when the CV hub end boot fails, you get black grease around the inner rim of the wheel, and often in the wheel arches.