Injecting air will only really work for coarse deads anyway, and even then it isn't reliable (unless I have been doing it wrong!).
With oil I usually inject it into the belly, how useful it is is a debate that will last forever though. I did most of last season using an oil injected bait on one rod and a normal bait on the other and I saw no real difference to be honest other than the mrs wouldn't go near me for a week after I used herring oil
mg: :wh
Some pike anglers I know don't feel confident using a bait without any oil added though and it doesn't seem to do any harm so why not. It's worth mentioning that I fish big, very clear stillwaters usually. No doubt people fishing rivers and coloured water will see different results.