I have no idea who you are buying the engine from so I can't say anything about their credibility for sure but I don't know any racers that even know where to get 6% nitro fuel.... not to mention how the engine was broken in causes biblical arguments between people that should know.

I'd buy new, but if you have to buy used I would get another engine from someone more in the onroad racing scene since its fairly small. rb offroad engines are good, try to sell it in the offroad scene and get more for it.

I broke an engine in once too quickly (warmup of a qualifier) and the piston/sleeve never loosened up like they should have so the engine never got the high end rpm it could have had until I got a new piston/sleeve for it and got the proper heat cycles through it. I am a pretty strong believer in the fact that the metals change when they reach certain temps which causes the right amount of wear and then something similar to a tempering process occurs and doesn't really change until it's damaged.