gear box oils and grinding thru sinchros

[danielhobby]

did a biggish weekend recently in the wonnangatta and discovered a problem in my 2001 gu petrol gearbox when in low.I was pushing it up a 35 degree ish hill in low third and it wouldnt pull so rushed it into second low,hit the sinchros and ground the hell out of second gear,ok, slow it down a bit allow a little more lead time and a slower shift and it does it again,find myself in first low and spinning trying to regain traction and momentum.this is first time out in this truck(fourth 4x4)and im wondering if the gu box is just slow and heavy and i tackle everything in second low or will a different oil in gbox allow quicker shifts and a more reliable quick shift?
what oil do most people use in box??