Had my Muscle serviced recently , Of course HD use their own brand of Oil for the service, would it be an issue if I topped the oil up using Mobil V  twin.

My intention is to eventually Change the oil to Mobil V twin, but in the mean time I don't want to waste the new HD Brand Oil put in during the service..............

if in doubt, get another filter and drum of oil and change it straight out.

$50 for the lot and there is no issue with mix

You should never mix oils, as they are constructed differenty and can react to each other.

Ok, because of the conflicting information rang Mobile Lubricants & Specialties Technical Assistance, Spoke to a Peter, said 'Not a problem mixing the two oils together'.
As long as they’re both Synthetic, it will be fine...............

Mixing full synthetic and mineral oil together isn't the best idea but mixing oils of the same general type (mineral with mineral, synthetic with synthetic etc) isn't going to hurt anything.