I've always just used the recommended plain jane regular NGKs and never had any problems. They're cheap and readily available, about $2 each at most of the auto parts stores around me. I swap them out regularly about every 4K miles. I don't feel $4 is too much to spend every four thousand miles. For me that's only once every year or two. By then I can usually detect some wear on them. It's not easy unless you know what to look for. Looking at one plug by itself won't readily show it. You need to compare the used one side by side to a brand new plug. They burn very clean and the edges on the electrodes remain sharp, so they always seem to look good. But, what happens is the center electrode gradually wears shorter. You can usually see this in a side by side comparison but won't notice it looking at one plug by itself.
I've found this old engine design and the wimpy stock points ignition is tough on plugs. While I still had points, I would sometimes see plug wear in as little as 3K miles. Now with my Pamco, it's better, but not much, lol. I always keep spare plugs on the shelf, obviously for replacement in case one craps out or fouls, but just as much to provide me with a comparison "tool" for checking the wear on my current plugs.
I learned about this form of plug wear many years ago on one of my BMWs. I took a trip to a weekend rally a couple states away. On the way home, the bike seemed to struggle and strain a bit at highway speeds. When I got home of course the first thing I did was pull the plugs. They looked just fine, as always. Then I checked my maintenance log and saw those plugs had been in there for nearly 10K miles. I compared them to new ones and was amazed how much shorter the center electrode had worn down on the old ones. I installed new plugs, took a test ride, and the strained highway cruising was cured. So, since then I don't push my plugs much past 4 or 5K in any of my bikes. Sure, they still function, but not as well as new ones.
