The 6 hour label on the bottle is intended to be for rain - which is different than scrubbing one down and washing them.
Pour on will chaff their skin a little.
So, what I was taught to do a long time ago - and still do because it works - is pour them last thing in the evening before turning them out of their stalls and then I wash them the next morning.
I strongly prefer Cydectin over Ivomec - it doesn't chaff them.
I use pour on, but you aren't going to get an injection site reaction with the injectable. That occurs with some vaccines and antibiotics, but I've never seen it with the wormers.
We use Cydectin once a month all year. The best thing to do in my opinion is alternate it with Ivomec or Safeguard a couple of times a year just to make sure you are killing anything resistant to your primary wormer.