Do you know something that I don't? Very interesting that you said that. I rebuilt the engine and found red grease had been used. It made splitting the cases quite easy and it wasn't leaking so I used it when I put everything together. I have no idea if they had been split prior to my getting the bike, but do you know if Honda did this?
Well, I'm NOT all knowing about motorcycle engines, but to my (limited) knowledge, most OEMs use sealer between the case halves. The Japanese all seem to use some variation of Threebond sealer; Hondabond, Yamabond, Kawabond, Suzukibond. Threebond makes various types of sealer, but the most popular type is a non hardening type acrylic sealer. Threebond also makes RVT type sealers too, and some OEM's have used that as well. Common thread, though, all use a sealer of some type. Threebond 1184 is the current flavor, but again, there are others too that will work just fine.
The issue with using grease, is that over time, the grease may wash out. At that point, there will be a leak path outside.