Originally Posted by JSP
Hasn't the gear box oil got to be marine grade?
Search here - there are differing opinions, but most people do use the marine stuff. You'll get that in any engine dealer/yacht chandlers.
As for the impellor I would definitely try to change it on an engine that is as old as that. It only costs about £15. There is a good thread here by Richard B on how to do it. Probably your worst case is you can't get the bolts out to get the leg off etc. If that is going to be a problem better to find out now.