Firstly, I learned that Yamaha would print me a manual, and ship to a US address only, for about $75 (I am in Scotland, but bought motore hen in the US)
Fortunately, OutboardExpert.com sold me a shop manual over the Internt for $13.50, no delay.

Secondly, the local Yamaha reapir shop in Oban advised me ngat the oil I had seen drippign when lifting the motor our (one time only) was prabably just unburned fuel, and that the only useful check on the conditon of the bottom shaft seal was to drain the lower casing oil and determine whether it was whitish. We did so, and found clean oil, so just replaced it.

No problem and a VERY helpful Yamaha dealer in Oban.