i used those very same plugs in our family van hoping to get approx 2 yrs or 15,000 miles out of them. didn't quite make it that far with the last set as the engine started running poorly. i switched back to NGK's which, as steve points out cost 1/4 the plats....
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"