Also, if there is nectar to be had they will often prefer to go out and get it, and the syrup in the feeder will be left.
If your brood box has only a couple of frames to be drawn out and you are putting a super on, I'd leave the syrup off for now. I was with one apprentice this lunchtime and the unmistakable aroma of (ling) heather honey came wafting up from a couple of her brood frames. What?! Yes, the newly occupied super on top had a rich, dark honey in some frames and I think that the bees, having had a spurt of brood raising recently, moved last year's heather honey out of storage in the brood box to make room. This year's heather is within flying reach for these bees, but it aint flowering yet.
If the brood box remains light at the end of the heather season, consider feeding again after taking the super off to ensure that they have enough for the winter. Thicker syrup is fine at that point or, if you can get some, bakers fondant can be used straight on the top bars. The advantage of the latter is that it can be left on in that state and topped up as required later in the winter. Doesn't have to be dried down, safe to leave, in the right place for them.
all the best
Gavin