2000 Chevy Malibu Green sludge on oil cap

Usually when there is coolant getting into the oil, you will get a creamy white / light brown build up and you can see it on the dip stick.

Since you only have it on the oil cap, chech to make sure the PCV valve is good and not sticking shut. That can cause condensation in the engine. Also, I have to be honest; I never saw a green build up. Keep an eye on the coolant to see if you are using it. Also, if the car has dexcool coolant (pinkish color) and that is what is still in the vehicle, don't mix any other brand with it.

Let me know what you find.

Joe