I have been losing coolant and do no think I have a leak. My car is a 3.8 engine. Just recently I have been seeing a lot of gunk under my radiator cap and at the top of the radiator. I have used 50,50 coolant mix I mixed myself, but used tap water which I hear is not good. I have changed my oil recently and it did not look like "chocolate milk" and I don't think it had coolant in it. Here are pics. Should I try to flush my system and replace thermostat while doing so so it get it all done at once? What is the best method of flushing? Thank you