Any 75W trials motorcycle gearbox oil. there are plenty of manufacturers around that are well known Putoline ,Silkolene, Rockoil, GRO, ipone,NILS Motul .....if you want something super dooper then you could try Putoline Nano trans GP ....It uses Nanny McPhee technology to increase its retail price.
if you want to find out what a top british rider used in your year of bike call up Splatshop [Sherco Specialists]and ask what they use in the gearbox....... they sell and ride Sherco's so any info is from the organ grinder.