I recently installed Bosch Double Iridium 0 242 230 534, since NGKs weren't available in 5/8 and I own a 5/8 socket only. Can't see any problems with them after several months of driving in different environments. Gas mileage and performance as before with the NGKs. As a side note, the old spark plugs were quite lose and oil/fuel had collected in the wells. After cleaning the wells and new plugs tightened to spec the wells remain clean.

 

That goes along with 50 to 100 miles in town low speed and rpm's light throttle driving carbon buildup in the combustion chambers.
Getting onto the highway with no traffic (cops also) and stirring the stick hard in 3rd and 4th past 100 mph before 5th gear will cause a 4-5 seconds a little puff cloud of tan and gray exhaist matter which clears up quickly. These pushrod LR engines have a habit of getting loaded up and need to be driven hard removing carbon buildup. Even a short 15 mile highway run resulting in a little smoother idle and overall better low rpm running manners.

I always questioned the flush center electrode vs raised like a normal plug not able to burn off as quickly as well not able to find and read the silver ring near the top of the center electrode. Altering voltage output from the TPS unit made adjustable to the ECU fueling properly the Mark Adams "Tornado" chip......~~=o&o>......