heres some similar info from neo buggy

Differential Settings
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Differential's are used to allow the wheels on the same end of the car to rotate at different speeds during cornering.
If diff's are set to tight or are "locked" the tires will fight each other as the car is turning.

Front Diff Looser than Rear Diff
Steering response increases
If rear diff is to tight, the car will understeer into the corner, and will cause loss rear end sliding out of the corner.
Could cause oversteer while entering the corner (depending on other settings)

Rear Diff Looser than Front Diff
Steering response decreases, stability in turns in increased
Could cause understeer or "push" while entering the corner (depending on other settings)

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Assuming we are talking about standard gear diffs. Not Torsen or Spider diffs. When the diff has very light oil in it, the outdrive cups will spin very easily in opposite directions.(diff action) So when the oil is very thick the outdrive cups will spin with a lot of resistance. This is important when one side of the diff becomes unloaded. ie, when one wheel looses traction.

This is most important in, but not restricted to, cornering. When turning the outside wheels have to spin faster since they have to travel farther than the inside wheels. The rear wheels are fixed pretty much staight so easier diff action is more important than in the front. The front wheels turn with the angle of the turning radius and the inside wheel usually turns sharper than the outside so on the front you need less diff action.
In the rear the lighter the oil the less power there is to one wheel if the other lifts or looses traction. The heavier the oil the more power to one wheel if the other wheel looses traction. Most important when turning because usually the inside rear wheel tends to unload. If you have too much drive power to the outside rear wheel when that happens then the rear end breaks loose.
In the front you want both front wheels to pull the buggy around the corner. When cornering chassis weight tends to transfer to the outside front wheel. So you want it to pull hard even if the inside front wheel unloads. So in the front the heavier the oil the more the outside front wheel will pull even if the inside front wheel unloads. This makes the buggy turn quicker under power up to a point. Too heavy wt oil can cause the buggy to push if there is not enough traction to the ground.

As for the center diff. It's still pretty much the same idea. The lighter the oil the less power to either end of the buggy. The heavier the oil the more power to both ends of the buggy. This is mostly important for keeping the buggy stable. If the center diff has light oil in it and one wheel at one end of the buggy looses traction then power to the other end drops off too. This momentarily slows the buggy down and lets the loose wheel regain traction. Heavy oil keeps the power to the opposite end of the loose wheel.

So, most of the time the rear wheels will loose traction more often than the fronts. You want the rear diff very light so when one wheel breaks free the other wheel stops pulling so the buggy does not swing around. It lets the loose wheel regain traction. In the front you want the wheel that still has traction to keep pulling when the other front wheel looses traction. All of this, most of the time, works out to a basic starting point of 3000/5000/1000 wt oil F/C/R.

If there is a lot of traction and you can put a lot of power to the ground then you want to stiffen up the diffs more, like 5000/5000/1000 or even 5000/7000/1000. If the surface is really loose and there is very little traction the you want to soften up the diffs. More like 3000/3000/1000 or so. This all works out to keeping the front end in front and the rear end in the rear. This is a generic setup for standard diffs. Torsens and Spiders work a bit differently. Thats a different story.