FYI:
The original wires were steel strand core. I expect this is for durability when screwing in the plug cap posts into the end of the wire. Steel will hold up better under the stress, friction, and vibration, and the resistance difference between copper and steel is miniscule in this application. Sales based info claiming lower resistance for copper, while technically true, is a sales tool rather than any real echnical benefit. Even it it were an ohm per foot difference (it's lower than that), you are only using a short distance wire in both cases and then inserting 5000 to 10,000 ohms in the path! A drop in the ocean.
It really doesn't matter where you put the resistance in the coil output. It can be in the wire, the plug cap, the spark plug or in combination. The total resistance in path to each spark gap should be about 5KΩ if you are running colder spark plugs (such as D8EA), or 10K if you are running hotter spark plugs (such as D7EA). The determination is based on spark electrode erosion factors and power consumption/heat generation of the coil and spark electrodes.
In theory, iridiums eliminate (or greatly reduce) spark erosion, IF both electrode ends of the spark plug are iridium, ground strap and center. This is because the erosion is polarity specific, and the SOHC4 system provides opposite polarity to half of its spark plugs.
Graphite is quite brittle, and therefore a boon to the automotive industry, due to their frequent replacement requirements, particularly in the high vibration environments. Even in cars, they are routinely replaced every 5-10 years. This is the reason why the type is so readily available. High turnover and profitability in distribution. I expect they are cheaper to manufacture, as well. But, I haven't researched that particular angle.
