If you get an oily/grease mark they need to be done again, as you know the dosage is small so any leakage will diminish the protection, I always use Nobilis for this reason, I hold the bird and my wife vaccinates being an ex-nurse, She takes ages to get it right and it bores me with the time taken, I always check next day and any with an oily mark will be done again, this is the only time I will use the opened bottle again, which would have been in the 'fridge overnight