the thingy is a hydrometer (
) and they are cheap at any parts store or walmart. dont mix anti-freezes, ones that can be mixed will say so on the jug. if you coolant still has good color, i wouldnt bother flushing it now. might as well wait to next fall before it gets cold again to add fresh stuff. when you buy it, use what you prefer, though if you have a vehicle that is under warranty be careful you dont use something that voids this. also dont buy premixed anti-freeze or you are paying for half water.