I have been running -7’s in my 340 for a few years. After pulling the plugs a few times in my now 340/420, I decided to order the -5’s after seeing a lot of buildup on my plugs. I used to run the -5’s when I ran iron heads, but was told to run the -7’s when I switched to aluminum. They should be here next week so I will throw in the -5’s and see if they clean up a bit.