I don't know that I've read Tekno specifically recommending changing diff fluid after a short term and gears have a chance to break in, but it's not a bad idea. I normally open up the diffs and have a looksee at the fluid after the first 12 packs ran through the RC. Yeah, almost always the fluid is grey-ish looking. I'll change the fluid and then I'll be back to my normal 24 runs maintenance schedule (+/- as I see fit, or needed). Everything depends on the thickness of fluid, and driving style.