Regular heating oil won't harm it. The new nozzle may not be the right spray angle. Or may be a bad one out of the box.
The oil tank is on a two line pipe system. Im thinking of getting another oil tank with a one pipe system. The tank is kind of old and also putting new tank in diffrent location.Is your burner on a single or 2 pip oil line. If 2 pipe, there may be a slight air leak in the supply line and that can cause what your having.