I use an OS p7 plug in my GRP .21 tuned (I just like to run medium plugs). I have used a p7 plug since the engine was new and have never had a problem. This engine now has 5+ gallons of fuel through it and it still runs strong. Also I notice no difference when I switch to a GRP plug. Personally I like OS plugs over any other brand. In my opinion they last longer and tune easier. I don’t know what the big deal with non GRP plugs not sealing as well in GRP engines is.