I just looked up the manual for your car and I confirmed that it has a sealed diff:
https://jlbracing.en.made-in-china.c...AGE-SEVEN.html

Personally I would used diff fluid just to play it safe and ensure your gears are properly lubricated and offer some level of tuning to make your car easier to drive.

You can use any automotive grease, I like to use Green Grease brand for all my rubber seals to prevent leaky shocks/diffs, but using the same grease on the planetary gears (as the manual shows) will let your car "diff out" and feel very inconsistent (twitchy) in corners. Grease will be more likely to gum up with debris and cause your gears to wear out more quickly.

For a Monster Truck I would go with 15K Front, 15K middle and 10K rear fluids in your diffs to start out with and then use this guide here to make any adjustments if desired:
https://ptrcracing.com/product-category/diff-oil/