My IS350 was on the original, Denso spark plugs for 100k miles. Original owner couldn't be bothered to replace them at the recommended 60k interval. I replaced them with NGK Ruthenium plugs and immediately got a 2 MPG bump in gas mileage as well as a silk smooth idle. The idle was a little rough previously. Now, I've got about 25k miles on the NGKs and I've been noticing the idle getting rougher and rougher. Average gas mileage is still better than with the 100k old Denso's. I just looked online and I realized that these NGK Ruthenium plugs don't have a mileage associated with them. It just says they are supposed to last even longer than iridiums. Anyone else have any experience with NGK Ruthenium plugs, and how long they last?