An observation - 90w gear lube is pretty difficult to find in the US. I tried both a standard type Multi vis 75 / 85 and a Synthetic 75 / 90 w (Mobil 1) and kept loosing lube between changes. Performance deteriorated, I changed to Dr Pulley sliders and an Athena Kevlar belt.
Performance kept going down.
Finally I found that Lucas is selling a standard blend gear lube, multi Vis in an 80 / 140w configuration and I splurged 8.50 US for it. At first, it ran a tad on the stiff side. Then after a few hundred km, everything seemed to click, noise level went down, performance went way up and gas mileage was again approaching 68 mpg.
Happy - Satisfied.
Prior to this, I had noticed a huge build up of black greasy residue inside the transmission case - partial belt dust but really oily. I cleaned it out with brake clean and added about 1 cc of Brake fluid to the final drive, pushed the bike around for about 40 km of riding, then changed to the Lucas gear lube.
It seems the lighter weight lube, could / would / did push pass the seal on the clutch side of the drive and it started to weep into the belt / transmission case. This fouled everything and I was constanly loosing about half my gear lube at every change. The Brake fluid will expand the seals back to where they should be. I've used this trick on fork seals when I lived in the SW deserts and in Australia ... otherwise you end up replacing them two or three times a year. The trick is to only use it for a short time, the seals swell up - then drain the lube and replace it with the heavier lube.
If someone can counter this - as to why I shouldn't use this heavier lube - let me know. Please!
Dion