Hi Paul,..as a M90 newbie (12-12-19)..a few thoughts...most normal sparkplugs are designed for gap tip operating temps around 700c (1300) degs F !! to promote self-cleaning of soot & other fouling electrode deposits by careful matching plug heat range to engine conditions. I don't think our o-ring'd M90's are running anywhere near that preferred sparkplug tip temps.There might be some soot reduction burn off effect if we could get the plug up to a higher temp by thermal insulating it with a nylon type washer (instead of the stock copper washer), thin thermal insulating paint coating on the sparkplug threads (leave a clean single turn for electrical contact) & replacing copper cyl head gasket with a suitable nylon thermal insulating type. This could also help get the valves & seats up to a better self-cleaning temp also. Btw...i never clean the sparkplug tip insulator with Anything Metallic (including brass, aluminum,etc) brush to avoid leaving permanent electrical conductive particle contamination !! If we're going to intentionally add oils to an otherwise near sootless burning clean naptha fuel (Coleman), we might as well help the sparkplug burn off as much needless wet crud as possible...just my newbie $00.03,..tom.