The manual lists two plugs for my 2002 90 ELPT 2 stroke. One was SG the other conventional. Surface gap became popular about the time Loop Charging became popular in 2 stroke outboards. Since they were introduced and have (as far as I know...at least up thru 2002 production, survived the test of time, they must be the best choice for all around performance. I had a set in my 1988 115 I6 Tower of Power for 7 years and never touched them.....it was Direct Charged, not Loop, but OMC was Looping at the time and using them.
Spark plugs have 3 basic requirements: Size, spark gap and heat range to suit the application. My assumption is that OEMs install what works for most conditions in a particular engine. Obviously there are exceptions and that's why the alternative conventional plug is listed. Not sure as to the reason why that would be the desired plug in a particular application.
