A cautionary tale about gear change shaft oil seals...
After rebuilding the 400 engine using all new seals from DS, I found it used to leak oil on the garage floor and over my left foot after riding . I could never catch where it was leaking from so I pressurised the crankcase with compressed air. At once, oil pissed out of the gearchange shaft oil seal. The new part from DS had been in stock for decades and had gone hard, and the sealing lip that contacts the shaft was split in a few places.
A new, rubbery, seal bought locally for 3 quid sorted it.
I couldn't change the seal with the shaft in place so the clutch had to come out so I could withdraw the shaft. Took me an hour or so to do so check any seals before fitting.