Leaking oil from the Gearshift

[born to be mild]

When youve got the seal out clean the shaft with solvent over the splines then wrap a bit of selotape around them to make sure that you dont damage the lip of the seal as the new seal goes over the splines. Smear some oil on the rest of the shaft then gently tap it home with a plastic hammer or a soft drift of some sort, a bit of wood will do.
All common sense really but whatever you do, dont go taking the engine out !