My first time servicing my truck (gearbox Oil)

Sorry guys just a quick one. I have a 3.0L 2002 GU Patrol and want to give it a service as i have dont about 5000km since i got him. As there is really no history i want to try and cover most things so i know where its at. Going to do an oil change, change the oil filter, diesel filter, and i want to do the gearbox oil but this is where i need help.

With the gearbox oil i dont know what i should be using? My gears going down from 3rd to 2nd, i guess youd say are sticky... and going into fourth is a little difficult. Even a little bit of a crunch. And idea what oil i should use or what i can do?

Also any more hints or trick or advise anyone could offer me would be great as i cant afford to take the truck to the mechanics and want to learn more on how to do everything myself.

Cheers guys help this makes sense... in a bit of a hurry this morning :-P

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