[RC-Monster Mike]

Here is a bsaic breakdown for you, Daf:
Front diff - heavy oil = harder pull out of corners, more push going into the corners. In general, you want the front oil a bit heavier than the back.

Rear diff - Heavy oil = more push everywhere, along with the tendency to "hang out" the rear on hard acceleration-generally, this is the lightest oil of the three diffs

Center diff - Heavy oil = less steering and more traction-OK for a blue groove track. Lighter oil allows the power transfer to "diff" better, which results in more stability on hard acceleration, less tendency to "hang out" the rear, and better steering into and out of a corner(particularly with electric power-a nitro setup will allow individual brake adjustment and the ability to tune braking-generally almost all rear brakes and very little front brake bias works).

Personally, I like very light oil in the rear(1000 or 3000 weight in 1/8 buggy)and equal weight oil in the front and center (5k for 1/8 buggy). This is for my particular track, which gets a bit loose and loamy. For a monster truck, heavier oil is almost certainly required (bigger tires = more "diff acting" force). In an electric Revo, I would start with 30k in the center, 7k in the front and 3k in the rear. Then, adjust accordingly for track conditions. This my personal preference. I prefer stability over traction(especially on a monster truck where traction is usually not an issue). I also like a lot of steering. :)