It is my understanding that Magnetos don't put out a lot energy, unlike EI systems. Thus you need a spark plug that is tailored to what they have available and to give a large enough spark to ignite the fuel charge.

Mahlon?
 
I remember in 1976 a new Lyc was 5-6 thousand dollars. Spark plugs were $20 ea. If they were adjusted for inflation they would be $75 ea today.

You can buy computer controlled/manufactured Unison plugs today for $16.50. Perhaps one of few items that has gone down in absolute and inflation adjusted dollars.

Just think, if Lyc built new, updated engines with the same economy of spark plugs you could get a new engine for about $4K. If you check the 2009 prices for factory rebuilds or new w/exchange you will not believe your eyes. These prices require you to turn in a usable core or add the core charge!

http://www.lycoming.com/utility/global-resources/2009-Aftermarket-Engine-Price-List.pdf
 
Without getting into a discussion about multiple spark discharge or spark duration, the only difference between a magneto generated spark and an EI generated spark is the voltage. The voltage is a function of the amount of copper wound around the secondary of the spark coil. With a mag you might see 15kV, and perhaps with an EI you would see 30kV. As far as the wire is goes, the impedances at those voltages are not of concern so there's no reason why one couldn't run resistor wire with a mag as long as you can figure out a way to terminate it at the mag. The airboat guys make harnesses out of 8mm auto wire to work with auto plugs and mags. I have a friend who owns P-51D "checkertail clan" that runs Bosch mags with automotive wires and automotive plugs, and he changes all 24 plugs every year, and buys them at autozone.
 
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There is an RV-7 at CYYJ using auto plugs with mags and in three years of flying he has had no issues. He has no noise issues.
 
spark plugs and Slick mags

As someone that very nearly got to buy a new Slick mag ($735) while running aviation plugs, with one that failed open, I've read up on the subject of late. Besides addressing the various heat range, reach, resistance value, and necessary gap issues, auto plugs may work out fine. But a field test that fails may cost you a new mag or unscheduled engine work. Unless the mag/spark maker mfgr and engine mfgr agree on a certain plug type, I'll pass on the auto plugs. Enough variables out there to not add another one. But as I've also recently found, apparently mag/plug emotion runs high, so sure there's lots of other strong opinions out there.
 
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