The 4 stroke plugs have to stay hotter because they don't fire on every stroke. OS's new plug chart is a little confusing. The A3 is touted as a economy #8 and a tad hotter............
http://www.osengines.com/accys/glowplugs.html
The chart is wacky The A3 is listed twice for air and surface . In the air it is positioned toward medium and car the hottest. If the average car user puts an A3 and their high nitro fuels it is rated as plugs per gallon. All older charts rate an F as HOT............
The # F is listed as the coldest airplane plug , but yet the # F is interchanged with MC14 listed as very hot and Fox Miracle as hot . Every manufacturer of 4 stroke plugs list them as med hot or hot but OS.........
http://www.scootworks.com/rdrc/gloplugs.html