You may have a blown head gasket. Have it pressure tested. The head gasket keeps the cylinder head sealed from the water that circulates through the heads to keep them cool. If the gasket gets a hole in it, then there is a gap in which oil can get into your coolant. Make sure what you are seeing is OIL in the coolant first.
On the coolant overflow bottle
Adding coolant to a 2003 Pontiac Grand Am is very easy to do. The coolant can be directly added to the radiator. Make sure the engine is not hot before adding the coolant.
Before changing the radiator on a 2001 Pontiac Montana the coolant must first be drained. After the drain remove the radiator mounts, fans, and hoses. Pull the radiator upwards.
You don't have a cap on the radiator. The cap is located directly on the coolant bottle
Add the coolant into the coolant reservoir. Be sure the engine is cool when you remove the cap off the reservoir.
Your radiator is probably leaking transmission fluid into your coolant. Trans cooler is built into the radiator. Replace the radiator.
The coolant capacity of a 2001 Pontiac Grand Am is 13 quarts. This includes the coolant stored in the engine and the radiator.
On the lower driver side of the radiator!
its a dance party
On the 3.8 V6 it is below the throttle body on the front of the engine. You do not have to drain the radiator, just remove and replace. Then top off the radiator coolant.
The radiator on a 1999 Pontiac Grand Prix is flushed by draining the coolant, filling the system with water, adding a liquid flush to the system, and running it for a few minutes. The system is then drained, flushed with fresh water, and refilled with coolant.
There is no radiator cap. The cap is on the coolant resivuar bottle on the passenger side under the hood next to the firewall