Unless I'm missing something, I thought Rain-X was a treatment you wiped on the windshield, not added to the washer fluid.
I doubt it's going to hurt anything, but to be safe, it might be a good idea to use a turkey baster to suck out as much as possible before the engine goes through a warm-up and cool-down cycle. When it warms up, the coolant is going to expand into that tank. When it cools down, it will draw the liquid from the tank back into the rest of the cooling system. If the Rain-X says it's made from a petroleum product, that can rot the rubber radiator and heater hoses from the inside and cause a failure.
As long as we're on the subject of wrong fluids, be absolutely certain you never ever add anything to the brake fluid other than fresh clean brake fluid. Any petroleum product like engine oil, power steering fluid, or transmission fluid will destroy all the rubber parts within a few days. That will result in a REAL expensive repair; probably more than the car is worth. We read about people doing that much too often.
Friday, December 6th, 2013 AT 7:46 PM