If you pick up your truck and spin one tire the other one should spin in the opposite direction. What the gear oil does is limit or enhance the ability of the tire to spin in the opposite direction at various rpm's and power inputs.
The thicker the oil the less the tire will spin in the opposite direction. Meanwhile the thinner the oil the more freely the gears spin.
The reason for using different oils depends on how or where you run. In a tight area with lots of corners you will want to use a lighter oil so the truck doesn't spin out from both of the tires applying power. On long straight aways and in a drag racing situation you want both of the tires to receive power so you will use a thicker oil.
Unless you race competitively you should probably just use grease in the diffs. That is what most of us use and it will work for the occasional race.

I hope this makes sense,



rolland