Simple rule of thumb I have been thinking about seems to be go thinner the more grip you have and go thicker to impact the wheels that are spinning...

If you focus on the front and centre diffs as they are the ones likely to unload the most losing you drive, going too thick on the rear diff will stop it unloading but at the expense of traction...

ie if the inside rear wheel spins up, you lose drive thru a corner slightly, if you go thick on the rear diff then the "axle" becomes more like a Kart, and when both wheels are driving then you can break the traction and cause violent oversteer...

Its worth remembering that grease acts differently to oil, as the oil has some "flow" or viscosity ensuring it is always making contact with the bevel gears, grease will tend to be flung off the bevel gears and only centrifugal/centripetal forces acting on it due to the diff spinning will put it back in contact with the gears, in essence it acts like a thin oil offering very little resistance and therefore not "tightening" the diff up..

Torsen diffs work differently in that they sense which wheel is turning more slowly and apply more toque to that wheel as long as both sides of the diff have some grip, if one side unloads then the diff will spin up like a normal diff...

LSD's or limited slip differentials are the ultimate diffs as they will not unload like a torsen or a bevel gear diff... they allow the inside wheel to turn faster for grip while cornering, but if the inside wheel slips, then they wont over drive it and transfer power away from the outside wheel... if the inside wheel loses grip then they lock up maintaining drive to the outside wheel...

The downside of LSD's is that with a lot of power applied they can cause the outside wheel to lose traction as well and this will cause sudden oversteer (rear diff) or understeer (front diff)

Adding thicker oil to a diff is an attempt to make it act more like a LSD, but it will not compensate for ham fisted throttle useage

Typical uses of LSD's in full scale are on the front wheels of FWD cars to stop you lighting up the inside front wheel in hard cornering, sort of mechanical traction control... they are also used in 4WD's to ensure that you get maximum torque to all 4 wheels in very slippery conditions...