I'm sure using oils has caught me more fish over the last few seasons.
Go for ones made of fish like Richworth mackerel oil or some of the excellent flavours ET now sells on his website.
If you buy fresh deads - say from a supermarket fish counter, or wet fish merchant - inject a bit into them before you freeze them.
When they're frozen solid, poke a few holes with a needle around the bit where you jabbed the flavour.
Useful enhancement in coloured water, when pike will be hunting by scent.
Otherwise put a few drops around the gills of a whole bait, or even just brush or dribble it on.
It definitely works for me, boosting the scent trail a bait gives off increases your chance of catching.
Sure, some of it comes off in a big slick when you cast a bait out. But it continues leaking flavour you can't see and pike will often come up off the bottom in coloured water and they have scenting pits underneath their jaws - look at the next one you catch - so trust me, it does work.
I've literally had baits out for ages without a take, added some oil and had a take within minutes of re-casting.