2000 Volvo S80 Engine runs rough

I dont think it was the fill up that started this but just bad luck and a coincidence the misfire actually started after the fill up? The flashing check engine light indicates a severe misfire that can damage the catalytic converter if you keep driving it. If you have the engine control module scanned for codes it should tell you what cylinder is misfiring like code PO301 for cylinder #1 PO302 #2 cylinder misfire etc. Most likely problem will be an ignition coil that has failed. But from time to time a fuel injector will fail as well? If this is the twin turbo inline 6 it will be very difficult to keep the engine running to try and remove the coil slightly up to listen for the popping sound indicating the coil is indeed firing and working properly. Whatever cylinder the misfire shows on the sanner, remove the coil and swapp it with another coil and make note what cylinders you swapped with. Clear the codes and start it up and see if the misfire followed the suspected faulty coil. If it did you now know the coil is actually bad. If code comes back on the same cylinder, if you are mechanicly inclined, swapp the fuel injectors and do the same procedure. If misfire follows the suspected faulty fuel injector, replace it. Good luck and keep me posted.