Diff oil leak

[NickArg <NickArg@domain.elided>]

My stag has a steady leak from the front of the diff. This occurs even when
the car 
is just sitting in the garage. I've change the pinion oil seal but this seems
to 
have made no difference. Any suggestions as to what else I should try? Could
the
quill shaft be worn?

I'm also about to change the oil in the gearbox. The manual states 75W for
drain 
and refill, but this doesn't seem to be readily available. Can I use 80W/90 or
90W?

Comments please

Regards

Nick

76 Mk2 Manual/OD in BRG