I am so glad that i decided to change the oil in my gearbox
MGR advise that it is changed at 100,000 miles (less on an F & more on a TF) I know that mine has never been changed, dont think it has been topped up either, from the service records and the car has covered 57,000 miles.
The horrible black stuff in the lemonade bottle, in the picture below is what I drained out
and it is less than two litres after an hour of dribbling. The refill capacity of the gearbox is 2.5 litres so was about 3/4 of a litre down. The new oil is a light straw colour.