Hi Slash,
While you are that far into the workings, its worthwhile changing the sprocket oil seal as well
I have seen many of these leaking because the circlip has not been fitted correctly, it can be a PITA to get it solidly into place, check the sprocket for wear where the seal runs or take the opportunity to change it if any wear is found
Clean around the end of the selector shaft and layshaft bush ends and seal with a smear of your favorite goo
John