I am not a fan of injecting air - I would prefer to use pop up sticks or balls.
But injection of oils is a great tactic, especially when rivers or waters are coloured up.
No real science here, I just inject oil into the stomach/gut area of the bait and it will slowly find its way out leaving a great scent trail.
On stillwaters on a choppy day you can often pre-empt a take when the surface flattens due to released oil when a pike picks up the bait but before the drop off indicator moves.
Donkey's years ago I read in Coarse Fisherman of a pike angler (can't remember who) who'd performed trials using various oils and the very best he reckoned was eel oil. I ordered 6 bottles of Hutchinson's stuff, but could I get it up the biggest hypodermic needle I could buy? Impossible so I used to soak the deadbaits in the oil and frankly, eel oil smells just like a dog's anal gland. I don't reckon it worked for me except that I once used it on the air inlets of a car that used to park in front of our drive, he soon got the message.
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.
If fishing big stillwaters, it might pay to chop up some sardines and pre-bait a particular spot. Pike then get used to picking up the pieces (not too small) and start to visit the area regularly. It's pretty much clear that pike know the difference between species of fish and know that sea fish are not native to lakes, but it's a meal to them and they take it. They just have to be made aware that it's safe to do so and pre-baiting does that.
Without getting too far off topic I prebait with all kinds, usually whatevers cheap at the fish mongers, not so much to get the fish in but to give the fish that are present a bit of something to get em going and to get them thinkign that eating a half or quarter dead fish doesn't always mean being captured which hopefully means when they pick up my bait they wont suspect a thing!