My wife took the car in for an oil change today. Oil change folks alerted us to some brown sludgy coolant in the reservoir tank. After taking a look for myself it appears the coolant is brown and sludgy. Some folks have told me it may have oil in it, others have said it's a bad mix of old and new coolants. I've also looked at the forums dealing with intake manifold gaskets and head gaskets. How can I better diagnose this problem? Is it bad gaskets or bad coolant? How would i know if it's a bad head gasket or intake manifold? Thanks for the help!

BTW it's a 2003 malibu

I added some pics:
http://www.wagnersonline.net/fingersludge.JPG
http://www.wagnersonline.net/towelsludge.JPG
http://www.wagnersonline.net/towelsludge2.JPG

Is there any indication that someone has added conventional (green) coolant to the system rather than the universal or Dexcool? Has the coolant ever been changed?

The photos look like what a I had with the bad lower intake manifold gasket on an '02. Also saw evidence of coolant getting into the oil. (Made oil kind of brown/muddy if checked shortly after driving, but with your ffesh oil change it may take a while to appear.) The car was also losing coolant, and the level on the oil dip stick was actually increasing due to coolant getting into the oil.

You could try flush the cooling system and replace with fresh universal coolant, then watch for changes in the coolant and in the oil. Also, look for evidence of external coolant leakage along the gasket, especially under the throttle body and near the cruise control unit. But this may be hard to spot.

FWIW: The gasket failed on the '02 at about 80K miles and 6 years.

Not a super difficult DIY repair if you are comfortable working cars, just takes a good bit of time, patience, and some tools. However, it is a costly repair at a dealer or shop.