Not a lot. The NGK BPMR6A is a replacement for either. The "7" or "8" designates the heat range, so the 8 is a cooler running plug than the 7. If you are getting crud, get a hotter plug. If you are overheating, get a cooler plug.
http://ngksparkplugs.com/tech_suppor...x.asp?mode=nml