Someone's been over lubing the chain for 40 years and what goes in must come out so you've now got an ugly oil leak.
Put bike on mainstand.
Remove cover to reveal gearbox sprocket.
Push a tube into the lower chaincase.
Pour in half pint white spirit.
Wizz wheel round to circulate the white spirit and clean all internals.
Place empty washing up bowl under rear chain case, then
Split the halves of rear sprocket chaincase to catch all the gunk.
Leave for 30 mins then apply suitable quantity of 3 in 1 on the new dry chain.
Put everything back together.
No oil leak