Unimog Club UK

The 4x4 box looks like a good option. You can get much quicker reverse speeds than you can with the 6 speed.

Oil wise, SAE 80, 80w90 or 85w90 can be used. Just make sure it's not a hypoid oil.

Edit... Reverse in low range only is pretty vital. The 6 speed box can make about 12kph flat out in reverse, the 4x4 box in low 4th can do 30! You really don't want to be going that fast backwards!