Grease won't make it less reliable but you can tune the handling of the buggy buy adding different weight oils to the diff.
Heres some info on diff tuning from the X-Ray Xb8 manual:

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
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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

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Rear
Thinner:

increases cornering traction
increases steering into corner
Thicker:

decreases rear traction while cornering
reduces wheelspin