It is unlikely that the guy at the auto parts store has a degree in in Automotive Engineering or Thermodynamics. The guy who selected the correct plug for your engine is likely to not only have such a degrees, but also has a labratory to do engine testing and access to feedback from the field, essentially millions of miles of experiance with just your engine. Now I'm not saying the guy at the parts store is wrong, just not the guy I would take my advice from.

Unless you do a lot of city driving and those two cylinders were the only ones working to begin with, I doubt putting the next colder range plugs in two of the cylinders will have a noticable effect on your cars start perfomance. Colder range plugs are designed to dissapate heat quicker, they have a shorter insulator. Typically a colder range plug would be used for high speed highway driving. This plug would have a tendancy to fowl in low speed short trip city driving.

By the way why were only two plugs replced.

John Roncallo