OM602 Vacuum Pump Question

[Frizzizle]

Installing a new Pieburg vacuum pump. Instructions say to install pump, leave both vacuum lines disconnected, start the car and feed a bit of oil into the nipple where the brake booster vacuum hose connects to the top of the pump.

I’m not very familiar with the vacuum system but it seems wrong to me to introduce oil into the vacuum port of the pump, wouldn’t it just suck oil into the brake booster?