Oil Getting Into The Primary From The Gearbox.

Good photo Philip. 

You have one of the early primary inner cases with four rivet holes and a thin oil seal. 
The blue dot indicates a leak path from the gearbox through the plain bearings in the sleeve gear, 
​​​​​​into the primary chain case. 
Not usually a problem. Some slight leakage must occur because the correct gearbox oil level is about the centre line of the main and lay shafts. I guess it could be worse if the bike is parked on side stand when oil will cover the sleeve gear inner. 

The sleeve gear has two bushes. If one is missing, both are worn, or they are floating loose in the sleeve, then you will have more oil leak from the gearbox. 

Can see that the bush at the clutch end of the sleeve gear does not come to the end. It should. Running with a tight rear chain over-loads the sleeve gear bushes in 1st, 2nd or 3rd gear. The bushes partially seize on the mainshaft and spin in the sleeve gear. And the outer bush has a habit of drifting out of the sleeve gear and is chewed up by the clutch. Suspect you have a damaged bush, which may last for ages in it's present condition if you can accept the leak.
And don't run with tight primary or drive chains. 

Peter
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