the #8H is stock for the 04/05 machines....and the #7L is stock for the 06/newer machines... .so your dealer was kinda confused....
however....I have actually spoken to the NGK rep about the differences between the 'H' and 'L' versions....and to the best of his knowledge.... NOTHING... even though NGK has chosen to alter the product numbers on this particular plug...there is supposedly NO DIFFERENCE other than the heat range...... which is funny cuzz each number/letter in most spark plugs, specifically NGK, have meaningful reference... but apparently not in this case...
now.....my experience with heat ranges..... (ignore this information for 2-strokes...cuzz with 2-strokes... the proper heat range DOES make a difference in fouling)... however, with my race engines... which vary from 8cyl to 4cyl to twins to singles.... I have always had the best results (or no differences) by using the COLDEST plug available.......w/o any adverse effects on 4-stroke engines....such as fouling
heat range is the ability for the spark plug to dissipate HEAT......the colder the plug...the QUICKER the spark plug dissipates the heat from the electrode to the plug threads...which in turn is connected to the head and gets cooled via the water jackets..... the colder the plug...the quicker heat dissipates... thus they are designed for HIGH SPEED, HIGH DURATION engines such as LONG DISTANCE RACING... etc..... and the hotter the plug...the LONGER it takes the heat to dissipate and therefore the plug stays HOTTER/LONGER.... meaning less fouling.... however... to squelch a long term MYTH....this has NOTHING TO DO WITH STARTING.... cold plug/hot plug, it makes absofukinglutely NO DIFFERENCE in the starting ability of the engine.... ONLY the long term effects of prolonged use.... if you run too HOT of a plug with a high performance/endurance type engine (such as X-country racing) you can actually damage the plug and/or piston by maintaining TOO MUCH HEAT inside the combustion chamber... causing blisters on the plug and possible DETONATION.... the theory behind a COLD plug is just the opposite... because the plug electrodes remain COOLER.... the belief is that this can cause premature carbon buildup and FOULING.... .however, in all of my experiences... this has NEVER been the case with a well tuned 4-stroke engine....(cant say the same about 2-strokes however)...
this is actually a very long and detailed topic on the heat range of plugs....it basically has to do with the amount of porcelain and distance this porcelain travels to reduce/increase the heat dissipation of the plug.... HOWEVER... to make a LONG story short...or at least a little shorter.... bottom line is this...I like running COLD plugs in my 4-strokes.... and moderate to hot plugs in my 2-strokes.... the larger the 2-stroke displacement...the cooler the plug you can run... but with some of the smaller 80cc and smaller class bikes... hotter plugs seem to work better.... CR250R/CR500R can run a relatively cool plug w/o fouling issues.......dont ask me why.... just my experience...
so...I run the NGK IFR9H-11... vs the stock IFR7L-11... and I am still running the exact same plug for over 3 years now.... w/o absolutely no signs of fouling....the IFR8H-11 is an EXCELLENT choice for moderately built 06/newer machines not running nitrous.... for nitrous or modded 04/05 machines.... I recommend even going colder to the #9 such as mine....
