I use 30,000 WT in my XXL. The difference between that and grease is like night and day, Before If I turned the wheels fully and hit full throttle, the front inside tire would immediately diff out and grow huge, Now with 30K oil doing the same thing it just goes and the tire doesn't diff out... I haven't tried 10k because I'm happy with the 30k, but I would say on a monster truck, you want to go thicker than you would on a truggy, since MT's don't steer well when you are going fast anyway, you don't have to worry about losing steering to the thick fluid....