Hello!
yeah you got it rite..R is there for Resistance and W20EX-U and W20EXR-U surely have difference.......!
For reducing electrical noise for radio, sound system etc, many modern ignition systems require resistor plugs to stop electrical noise from interfering with the vehicle's on-board electronic control units (ECUs). If non-resistor plugs are used in place of resistor ones, the result can be malfunction Resistor spark plugs should always be fitted, therefore,where specified.
SO its better to get that W20EXR-U rather than W20EX-U
Regards