I make my own juice. I don't add sweeteners at all, though i did in the past, even though I typically use sweet flavors like fruits. I have no problem with gunking. I personally don't think the sweet flavors need actual sweetener and feel like all it does is make the juice it's self taste slightly less horrible if you accidentally get it in you mouth. I don't really notice a difference in the vapor. So you can have sweet flavors just not sweetened ones if you don't want to prematurely gunk up your coils.
I've noticed issues to varrying degrees with sweeteners, dark flavorings, and light but artificially colored flavorings. Sweetener typically being the worst.
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