The patented DENSO Iridium TT Spark Plug is fast becoming the standard for modern high-tech engines. 1990 Volkswagen Jetta 2.0L L4;9A. 1990-1991 Volkswagen Jetta 2.0L L4. GENUINE DENSO SPARK PLUGS .
These double Iridium plugs are amazing.
I immediately gained fuel mileage and reliability. They save more than they cost.
This is important on older vehicles (in this case, Jeep XJ) that do not have the efficiencies of newer, more advanced vehicles.
I also use this same plug design on my newer vehicles as well - and they do help.
I have not owned them long enough to comment as to service life, however.