Have a read of the Xray set up book
Click
EFFECTS OF DIFFERENTIAL OIL REPLACEMENT
DIFF OIL THICKNESS CHARACTERISTICS
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
differential because it overloads
More off-power steering
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