Oil in the coolant tank indicates a head gasket leak.
Perhaps the coolant was overfilled. It will expand because of heat or pressure and the excess makes it way to the coolant refill tank. Make sure your coolant doesn't have oil in it. In a worst case scenario you could have a faulty head gasket or engine damage that allows engine oil to seep into the coolant. But if you don't see oil in the coolant, then just drain some off.
Have the coolant changed immediately and you will be ok.
Usually a blown head gasket.
Engine? 3.1L V6 are notorious for intake manifold gasket leaks. get it fixed soon or your engine is at risk. My Lumina had oil in coolant and coolant in oil. If there is tan foamy stuff on oil fill cap, coolant is in oil too.
if there is oil in the coolant system it could be a crack in the intake, a ruined intake gasket, a cracked cylinder head
below the coolant tank in a black cover.
Blown head gasket or cracked head.
Driver's side, rear of engine. Between the oil dip & the coolant tank.
If it is an automatic transmission, the cooler in the radiator is most likely leaking into the coolant system.
Normally in the coolant reservoir recovery tank. If you have the supercharged model it is located on the radiator beneath the filler cap and oil cooler lines.
The tank holds 18 gallons of gasoline. Check the manual for other capacities such as coolant, oil etc.
Check your intake manifold gasket