Grease is fine for starters.

As you get more into the hobby, you will learn that you can adjust your buggy's handling characteristics by filling the gearboxes with silicone fluid instead of grease. Fluid is available in different weights. A good starting point for an Ultra would be 7,000 weight in the front, 10,000 in the middle and 3,000 in the rear (7-10-3 for short).

PS. There are many articles online about using fluids instead of grease..... suggest you read some for more info.