The Castrol and Valvoline (Aust) Netlube sites may help. The don't seem to list the model 45 as such (maybe just a little too old) but do have later model UTB 45 HP tractors (445). There might be a NZ equivalent to these sites which lists that tractor if a lot were sold there. Castrol Australia - NetLubeValvoline Australia - NetLube
If you can't find the U45 on any of the datateck sites (I can't) maybe look for the lubricants for the equivalent Fiat model (year & HP).