resistor spark plug question?

[Frijid]

I've always ran resistor spark plugs in all my old B&S push mowers, even though i've been told you don't have to. today, someone gave me a B&S 3.5 hp engine that they were going to throw away that was leaking oil from the head. While i was taking it apart to replace the gasket, i got the bright idea to watch the spark from a plug. So i tied the handle down, and put a new j19lm plug i had laying around the house inside the hole and watch it spark when i pulled the cord. When i did this, it had a bright blue spark, and it was noticeably loud. I then took out that plug and installed a new rj19lm plug and re did the "test." this time, you had to struggle to hear the "snap" and the spark was a mixture of blueish yellow. Now, i know that the resistor plug would have a weaker spark, due to it having built in resistance, thus limiting the spark it would put out. As stated before, i've been told you can run non resistor plugs in these old units, however the past few years, i've been running resistor plugs, with no problems. But i got to wondering, if the resistor plug is making a smaller, more weaker spark, what are the odds of fouling going from using resistor plugs? obviously, it isn't a stronger spark, so the non resistor is going to be more "self cleaning" by making a better spark, but would there be any chance you'd get more fouling from using a resistor plug? Logic states, that when you go to buy a new push mower from lowes, and it's a briggs, 99% of the time it's gonna have a resistor rj19lm plug installed already. The manual to my weedeater says to only use a resistor plug or coil damage could occur, so i don't really have a choice with it. but will running resistor plugs in these old engines cause any damage?