For the record, I switched to Penrite's Mild EP gear oil in the gear box
No more foaming, and it also quietened a gear wine in 3th, which was very noticeable when I ran it on engine oil.
One little problem left. After about 400 miles it started to leak oil at the chain sprocket oil seal. The sprocket was replaced, and to be sure, I polished the sprocket's contact surface, so I think it must be due to a bad quality oil seal.
Anyway, this means the clutch has to come off again