There are several threads on Gas Gas gearbox oil,you can spend hours reading them. On the TXT Pro some recommend ATF but there are some makes of clutch plate(DP is one of them) which the friction material will swell on the Diaphragm type clutch it leads to a hard pull clutch. On your bike which is a conventional clutch i would use 75W light gear oil such as Putoline or 5/30W engine oil(not car type but motorcycle specific) Many people also steer clear of fully synthetic oils too. The more i read the more confused i get!