If you aren't seeing any smoke, that would almost have to be a leak. Just look for wet oily spots on the engine. Put a piece of cardboard underneath the car after you get done driving it and let it idle for a few minutes. Maybe you just forgot to put one of the quarts of oil in when you did the change. A misfire can be caused my a lot of things. Faulty fuel or ignition components, timing issue, or fluids (oil or coolant) getting into the combustion chambers (you would see smoke). Pull out the plugs and look for discolorations first. If anything is getting into the cylinders, the plugs will tell.