I have used Iridium spark plugs in the 4/4 and Lotus for many years. After changing the head gasket, my son screwed in normal spark plugs in the 4/4. I have to say, the engine runs super well, the plugs cost about 3€ each and I stick with the normal spark plugs. Changing them every 20 to 30,000km or so is ok. I even think (but it may be my imagination) that the engine sounds more throaty and pulls a bit better with the normal plugs. At least it doesn't run worse. I switched to Iridium in 2015, two sets since then, but I had forgotten that the normal plugs were so satisfying.
BTW I saw a new video from "Harry's Garage" yesterday where he starts his beautifully restored XJ 12 for the first time. Very worth seeing. The technician says that modern cars have spark plug change intervals that are far too long and that he definitely changes the plugs after far fewer miles.
But I have found another improvement. 1.5 years ago, I installed a new coilpack as part of a troubleshooting project. It was not the fault I was looking for, but after 80,000 km the new coil pack seemed more powerful than the old one.
