All are slightly repellant I think, but the effect seems to wear off quickly, and they shouldn't be used daily on animals which self-groom or mutually groom. (Which means okay for sheep on a daily basis?)
I wear a citronella pendant when milking, and once weaned, we put a coconut oil and tea tree salve on the udder after milking, to keep flies at bay for half an hour (while the teat entrances are open.) These things help but are not 100%.