I'm building a vehicle which is based on the Hyper 7 four wheel drive
system.

Having seen a few in action, it seems to suffer from not having
limited slip differentials.

I've looked at the tuning guides for the Hyper 7, but it's fairly
obvious that they're aimed at racing machines, whereas I would like
the best cross-country performance.

Is it likely that filling all three diffs (front, centre, rear) with
oil as stiff as 50,000 would give a rigid enough drive train, and
would this have a dramatic effect on the tyre wear ?

I've even seen oils up to 120,000 available online, unless that was a
misprint, but that would have to have the consistancy of plasticine !

John S.