I'm nervous...

couple weeks ago replaced a cracked water heater valve--and of course I over heated the car when the valve failed(though not for long). This past weekend I changed tranny fluid, filter screen and gasket during my normal oil change; also topped off/checked ALL my other fluids. Today, the check engine light flags me! I checked the coolant level and there is a brown/black scum coating the inside of the tank and cap as well as floating in the coolant. Next move--check oil. Although I am already a 1/2 quart low, I see no water--though I have not drained it to make fully sure.

My first question is: if there is a leak somewhere, doesn't the coolant normally go to the oil, instead of oil to the coolant?

Second question, if the oil is getting to the coolant, does that tell anyone where the likely problem is? I do most of my own work, but a mechanic I aint.

I am at 178K miles with this car, which is in great shape otherwise (tranny is gonna need a little work very soon; 3rd gear is occasionally not downshifting--solenoid, maybe?), and can't give her up. In the last year, I put a k&n air filter, started using mobile1 and when the exhaust needed a new muffler, I went ahead and put a 2.5" TME exhaust on it. If anyone has some freindly advice so I can save my baby, please let me know!
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Cheers and Thanks!