i highly recommend global racing oil for all trials bikes! They do two types - gear trans 10w30 and gear extreme 75w. the extreme stuff is £20 for 1l and the gear trans is £10
but i have tried both and cant see any difference! they are the official oil supplier to gasgas,ossa and oset. in our sherco which has a similar to the beta we used gear trans which eliminated all clutch drag and selected gears cleanly. i suggest you try this first as they are the best i have tried. http://www.globalracingoil.co.uk/
http://www.globalracingoil.co.uk/gear_trans