Did you run the car with oil in the coolant?
If not, you may be able to use a fluid vac to suck the oil out of the tank.

Oil floats on coolant and will remain on the top of the surge tank. All but the oily residue should come out without too much fuss.

If you did run the car and the oil has cycled through the entire system, there are flush products available for this sort of thing. Usually they are intended for oil cooler failures and such but I guess oil in the cooling system is the same, no matter how it got there...

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2005 4.8is