Here is a chart I picked up awhile back see if it help's, applies to O.S plug's so not sure if it's any good for other brand's

OSMG6300 #0 Economy standard plug similar to the #8
OSMG6304 #1 Hot plug for low nitro (0-5%)
OSMG6308 #3 Medium plug for medium nitro (5-25%)
OSMG6312 #5 Cold plug for high nitro (25%-plus)
OSMG6316 #7 2-stroke engines requiring an idle bar
OSMG2690 #A3 "The new standard" for use in all 2-strokes, any nitro
OSMG2691 #8 "The standard" for use in all 2-strokes, any nitro
OSMG6320 #9 For general use in 2-stroke engines
OSMG2692 #F For O.S. and other 4-stroke engines
OSMG2688 #RE For the O.S. Wankel Rotary engines