I agree with the grease only position. I use Lucas X-tra Heavy Duty- pretty green color. I liberally applied it to the first, intermediate and final gears and pack more in behind the intermediate before putting the cover back on. I hear only the chain and the engine- gears not at all.
From time to time you will see posts with people talking about how noisy the Grubee gearbox is- older ones maybe, and without proper grease, surely, but I got mine last August and it is quiet with grease.
The Grubee Skyhawk II needs some "bodging" to get it right, but when it is set up- it is really quite good.
I use the Honda engine and it is a rev happy, mini-motorcycle sounding little beast. I love it.