The use of NGK plugs only may be just the fear of having to replace an expensive DI cassette, this fear led me to try; I found a used DI cassette, cleaned it up and checked the fluid in all the coils, installed a set of Bosch Super (copper electrode) FR 7 DCX, these are resistor type, applied some dielectric grease to the boots and installed the cassette. Have been running for more that two years now, no problems; the experiment went on and on until I just forgot about it, I'll leave it alone until or if it fails but so far so good. Lack of maintenance of the DI is what kills them, installing them dry alows arcing and corrosion to form inside the boots, always use dielectric grease, aditionally; when removing it to replace plugs, do not place it upside down, if you do, wait a few minutes after reinstalling it to allow the oil in the coils to settle back down otherwise they overheat and burn.