Putoline Race Gear oil is a fully synthetic, 75W oil developed for motorcycle gearboxes and shaft drive gears and is the  equivalent of Yamahas own shaft oil but at a fraction of the cost. (Typical Yamaha Dealer cost per fill of shaft unit is in the region of £15 - £20!)) Other oils may not offer the correct protection.
 
One 1L bottle will do approximately 4 oil changes.