I have a 2005 pontiac gxp that has been using oil and antifreeze but I checked the antifreeze and there are no signs of oil and also the oil dipstick has no antifreeze on it either.....I used about 3 quarts of oil every 3,000 miles and have had to replace about a quart of antifreeze in the overflow in the past 6 months.......

If you have the V-8 Engine, just totally disregard this post.

If you have the 3.8L V-6 engine, your oil consumption problem is most likely to be caused by the PCV Valve if you cannot find any external leaks. These are also known for the upper intake plenum leaking antifreeze. The ports in the manifold that go to the throttle body are made of very thin plastic. The EGR tube, with its hot exhaust gasses, goes right between them. This causes the plastic to heat up and crack, causing an antifreeze leak that goes directly into the intake and gets burned in the combustion chambers.

DORMAN makes a replacement upper plenum kit to fix this problem. The kit includes the upper plenum with thicker coolant passage walls, an intake manifold gasket set, a smaller-diameter EGR orifice, and a new PCV Valve. This kit will probably fix BOTH of your problems.

The DORMAN part number for your car is 615-180 Shop around...prices vary considerably.

Could be a head gasket problem ! it dumps into the cyclinder and exit thru the exhaust have a block test done good luck

Sounds like your head gasket is bad. What color is the oil? If the oil is greyish white then your have radiator fluid leaking in to engine. IF not then it is probably your head gasket.