I'm kinda new to the whole rc thing, I was running my outcastb4s this weekend and it started making a whiny/grinding kind of noise then had no power when I'd pull the throttle,. Thought it might be a loose pinion or spur gear but after pulling it apart i'm thinking it has to do with the rear diff. Read some stuff that they come almost dry stock just curious what I should use to top it up? And can i just buy stuff from any store or does it have to be something special from a hobby shop?