We have had yearly to 2 year plug changes with delco on the 496 was not uncommon. I do know this for a fact the Volvo 496 runs better with NGK s and one can tell the difference. The Merc with NGKs idle a tad better and hold up to better long periods of idling. I do know this for a fact as well.

The worst was Denso plugs in the 496 s Merc installed them at one time but fianlly got rid of those installs. Poor running plugs in the 496 s

This spark plug talk is very interesting . I once had a car that ran like sh*t with anything but Bosch plugs in it .
I hope the NGKs in the 502 are a good choice .

BUP, I'll run these for the rest of the year and give the NGK a go next... thanks.

My ProCharged 502 years ago was Autolite friendly. Would foul the NGKs on a reg basis... go figure! Overall I'm an NGK guy due to their success in my past endeavors.