I took my car in for a routine coolant flush. The mechanic reported oil in the coolant, which he said is a symptom of a blown head gasket (gave me a quote of around $2000 total to fix). This would not be worth doing given the value and age of the car (see profile info). Since the car has been running flawlessly for years, I didn't concern myself too much with it. This was right before I took a 1300 mile road trip across the country. The trip went flawlessly, and I got better gas mileage than I could have ever dreamed of with such an old car (over 30 mpg). Although, not long after the trip, I noticed the "Low Coolant" light came on, which surprised me since I had just gotten new coolant. With a cold engine, I could not even see any coolant in the reservoir, and I noticed the inside of the tank was a bit dirty. After driving around for a little bit, I checked again, and could see a couple of inches of liquid at the bottom of the coolant tank. It was greyish-brown, and appeared to be thicker than coolant is supposed to be (possibly more oil that the mechanic reported earlier?). I filled the reservoir back up with new coolant.
Any ideas about what could be wrong? It's clear that there is an issue here...since I lost almost all my new coolant over 1300 miles and the remaining coolant didn't look right....but the car is still running perfectly. I always watch the temperature gauge and it never gets very high. But I also noticed the oil gauge spikes all the way up instantaneously every 10 seconds or so.
Can I get by with flushing the coolant every couple thousand miles? The car isn't used heavily (65k miles over 15 years).