Lots of old wives' tales floating around.
Oil in the barrel WILL drop velocity.
So, push two patches of Cigarette lighter fluid down the barrel prior to hunting if you have oiled the barrel previously.
Then, go hunt.
Some simple testing at the rifle range can teach you volumes with your own equipment. On my custom barrels, fouling shot after cleaning, with no oil in the barrel, is rarely ever off more than 1/2" at the most. When I push a couple of tight patches on a punch type of jag through the barrel with cigarette lighter fluid on the patches, shots go in the group.
Not all barrels and loads produce the same results after cleaning or oil in the barrel from Storage. We learned volumes during competitive shooting concerning fouling shots. Often fouling shots that are flyers are just the shooter settling down, but we do not want to go there!
Shooting off of cheap flimsy Bi pods produces all kinds of skewed results....expensive lesson to learn.