At any rate, NGK customer support gave me a reply to this inquiry:
I have a 1978 Honda GL1000 Goldwing motorcycle. The owners manual calls for an NGK D8EA for a U.S. motorcycle, and an NGK DR8ES-L for a Canadian model. I have at times used either plug in my motorcycle. What is the technical difference between the plugs? Why are there two different plugs specific to U.S. or Canadian models? And are these plugs compatible with either machine, or is there a technical disadvantage in using one over the other. If either plug is compatible, why do they have different numbers? Any details that you could provide for me would be most welcome.
I got this reply:
The D8EA is a non-resistor spark plug in a heat range of 8, the DR8ES-L is a resistor spark plug and the –L stands for ½ a heat range colder. My listing shows D8EA to be used for a standard nickel spark plug or the Iridium IX DR8EIX for a high performance spark plug. The recommended gap is 0.028”.
The D8EA is obviously, at least 1978 technology. Has anyone had occasion to use the Iridium IX DR8EIX spark plug, which by the way is nearly 4 times the price of the D8EA spark plug, did you notice a significant performance change, and what would those changes be?
This topic has "sparked" my attention.....................Dave

PS....Just to clarify, the ad info. that I've read specifies 0.032" gap for the Iridium spark plug.