I've always been a tightwad on shields so I use nothing on them but soap and water. I take them off and wash them using dish soap and water, I lather the lense up and let it soak a bit, then wash off. I've also used folded paper towels soaked with window cleaner too, to soak off the dried bugs. Otherwise the bugs remain on them until I do wash them. Seems most any rubbing of any sort has cause swirls when I've done so in the past.
I've only bought one new shield for a helmet in the past 15 years or so, never needing new ones.
I do a similar plan with goggles, but use tear offs on the lense, since they're dirt cheap. I've replaced two lenses in about 12 years, using the tear off plan. The leses eventually get scuffed by dust that gets under the tear off.
That's the way I do it.
Ever get lost? You know, that good kind of lost - come to a dirt road intersection and you have no idea where you are or which way to turn? I like when that happens!
Mark - klx678
95 KLX650C w/Vulcan piston bigbore, 90 Zephyr 550