Most pour-on products only control sucking lice effectively. One exception is Ivomec. Its expensive treatment if all your treating is lice.
I would suggest two non-pour options to save the hide and clipper blade sharpness:
Lindane, properly dilute in a spray bottle and apply consistently on entire animal, repeat in 14 days (life cycle) May be difficult to purchase, so locate a senior herdsman or an old showbox. It used to be a standby treatment in rinse water. You will see the critters unlatch and drop off on contact.
Vetolice is oil based, but can be used in a spray bottle for consistent coverage. Label indicates ml or oz. rate according to weight and use a sprayer with volume measurements. In using this method, the product isn't concentrated on the topline. Oil based, so treat 7 days (minimum) before show, giving the coat time to return to its natural body and bloom.