Being oil fired makes no real difference, it still won't start condensing until the return temperature is below about 56 deg C.  The IR thermometer will be fine for measuring the return temperature, with one proviso.  It won't give an accurate reading from a copper pipe, but will if the pipe has a bit of paint on it.  It's to do with infra red emissivity and the way these things are calibrated.  They are set to read reasonably accurately from a matt light grey to white painted surface, with an e