According to this I found on this site I should thicken the diff oil in the rear:

There is actually lots on here about diff oil s if you search for diff oil s, but for those who may have missed the info here is a quick summary

Front
Thinner :
increases steering into corners (off-power)
if oil is too thin the steering may become inconsistent, especially it can lose forward traction (and steering) during acceleration out of corners

Thicker:
increases stability into corners during braking
increases steering on-power at corner exit

Center
Thinner:
front wheels unload more during acceleration
decreases on-power steering (reduces oversteer)
easier to drive on rough tracks
if a high-power engine is used you could waste too much power and sometime “cook” the oil in the center diff erential because it “overloads”

Thicker:
more all-wheel drive effect
better acceleration
increases on-power steering (reduces understeer)
better suited on high-bite, smooth tracks
car can be more nervous to drive especially if a high power engine is used - you might need to be smooth on the throttle

Rear
Thinner:
increases cornering traction
increases steering into corner

Thicker:
decreases rear traction while cornering
reduces wheelspin