The commonest problem is missing gearbox oil. Oil can leak out through the worn-out driveshaft seals. Check the oil level regulairly and if it's found not to be leaking anywhere but oil is still missing, it's time to check the driveshaft seals. This is a very common problem, especially on Series 1. Another, far less problematic, typical problem is the oil leaking through the plastic part where the gearchange lever enters the gearbox; however, this does not decrease the level signifficantly.