I have the exact same engine & the exact same problem. Found another thread where a guy says the upper gear shift arm rides in an oil bath, and that there are seals there that can leak. Here is his post: http://forums.iboats.com/honda-tohat...ak-305154.html
Doesn't sound like a cheap repair. I've ordered the manual for my boat & will try to figure out how to access those seals when I get it.
Let me know if you figure yours out & I'll do the same!