CovKid wrote:Just get an oil seal remover (Machine Mart do them for around £7) and you can almost flick the seal out. You just need to drift the new one in. Not too hard and no you shouldn't lose any oil (or not much). Just put a few rags down if you're worried.
I'd just add that it's not always easy to get the old seal out when they are surrounded by 20 years corrosion, even with the correct tool, especially with the input shaft in the way, it is also adviseable if it is leaking to remove the bellhousing and check the oil thrower behind the seal is not damaged and is still solidly fitted in the case as 9 times out of 10 if the oil seal is leaking it's because the oil thrower has been damaged and thus the oil seal can't cope with the oil load, it is also much easier to fit the seal (27mm socket as drift, oil the outer edge of the seal and clean up any burr on the case caused by the leverage end of seal removal tool or it may damage the new seal when fitting) without the input shaft in the way.
If you take the end mount off the box and pop a golf tee or similar into the breather hole you can stand the box on it's end, but unless the oil is new I'd change it anyway it won't do any harm to have fresh oil in there only good