tarmac is where ive always seen the highest temps, you will still generate heat on a tarmac surface simpley because of the grip factor and the diff is designed to allow wheel speed changes, in a plates diff or in a crappy atb heat will still be created in the oil, on the loose the car tends to 'skate across' the surface once moving hence the load on the diffs are less than on any tarmac suface, gravel is not know to be a transmission breaker , tarmac is !!

viscous can be a problem to themselves, needed heat to work properly but having to much of which they do generate a massive amount on tarmac causes the problems i discussed earlier to which doug is on about , not plated or atb.