So you believe selling oil from teh SPR will lower prices but filling the reserve will not raise them?
The price of oil has a lot to do with expectations. If the US let it be known that we were going to screw with the price of oil and knock it down through overt supply manipulation whenever possible, expectations would be for lower prices in the future and the price would fall.
There is also the matter of dumping suddenly and loudly and buying slowly and quietly.