well I think I finally got the true scoop on the Flushing machine at Toyota service centers from a 4th Toyota dealership service, its not really a flush because Toyota corp doesn't approve of CLEANING AGENTS to be introduced into their cooling systems its really a FLUID TRANSFER whereas the few ounces of old coolant left in the heater core are moved out of the system and replaced thru pressure of the machine so there wouldn't be any remnants of old antifreeze left at all, with the drain and refill there would be a few ounces left behind in the heater core, so a few ounces of old fluid mixing with at least 90% new coolant would harm anything I don't think so.