Well I have used Oramec in my goats, and have applied the 7 days withdrawal rule (as it is not licensed for dairy animals anyway), and we are still alive and kicking (goats and humans and pigs....).
The 60 day withdrawal period is on it as it in all probability has not been tested for residues in milk.
My vet is more concerned with injectable wormers during lactation, as he thinks the drenches are much quicker moving through the system, so I would always use a drench in preference to an injectable wormer.