My wife 1st car was a cavalier. I'd get 38 pretty regularly driving normally so that's pretty decent mileage. First thing I thought of was the thermostat as well. Nasty habit of going out and overheating =headgasket
Now that death-cool stuff is special. Organic and turns to jello when it s exposed to the air. I think it congeals when mixed with green and when it gets really old too. I'd definitely thoroughly flush it out. My old Tahoe had the boogers forming around the reservior. You could go back to it or go green but not together. It works as good as weedbegone at killing grass too.