if it only does it under load ... it might be the chain or the rear hub or sprocket jus saying but it is a normal honda thing
I was thinking the same thing, I took the side cover off and had a peek at the sprocket/chain and it was not worn at all, I thought maybe a tooth might be missing or the teeth wornout and the chain was skipping but all was good and tight. The sound is not comming from the rear of the bike (rear hub) , it's comming from right below me when on the bike and riding.
ya try the synthetic and see what happens and blled the master and slave cyclinder for the oil and i am pretty sure you can bleed the clutch out change all fluids you can to synthetic.
i had a honda 550/4 that did that and the man i got it from had put new gears on it, but it ended up being the chain was bad.
At first I thought the chain as-well, I tightened it.....loosened it.....oiled it.......ect, and there was no change in the sound that was coming from the trany area so I ruled it out. I would be shocked if the chain was the issue but thanks for the thought.