Most synthetic oils have additives that makes limited slip and lockers ...slip, not function properly (for a lack of a better word).... if you run a locking differential; use regular 80W90.
I even tried limited slip oil for my TrueTrac and it worked too much. It was grabing too much on the warm gears and limited my turning like if I was fully locked. It was also making scarry noises.
Stick with reg. 80W90... like the boys said... since you use your Heep to offroad; you'll change your fluids often anyways..
18 l buckets of 80W90 at Walmarde for $35 is what I use....