I was told by my mechanic to run my tank down low, then add the fuel. He did not give me a good enough answer as to why to do this, but I did it to be safe, and had no issues with the switch over, I ran my tank down to about 1/4, maybe a little less, but more then 1/8 of a tank and then filled it completely with the new fuel. I think it was more of a precaution then anything. Not sure what would happen if you mixed it in slowly, possibly not much but I'm not an expert in fuel's. It probably is just a slightly different amount of ethanol in the fuel, or it may be from a new distributor that may have new additives. I'd think as long as you are adding star tron and ring free you should be fine, hopefully others will chime in and maybe some may know reasons why not to do this, but I cannot think of any.