You cannot throw your own feces out of a moving vehicle.

The Law

Under laws regarding the transportation of solid waste, Oregon strictly forbids tossing human waste from a car window onto the highway. While this falls under the broader category of littering and biohazard laws, it was explicitly addressed after long-haul truckers and road-trippers got a bit too lazy to stop at rest areas. Keep your bio-waste inside the vehicle until you find a trash can.