How does a boiler pump circulate water

It depends on how much height difference more than length of hose. For a couple feet of lift, you can use a fountain pump. But if you need more speed and more lift, then a submersible utility pump will work. Note that the utility pump may not be designed for long term use, they are typically used for draining something that has flooded and then going back into the tool shed. And both expect that the other end is always open.

Depending on exactly how you plan to use this, a sump pump may be ideal to lift the water from a low barrel to a higher one, and then you can tap the higher barrel for your purposes, only opening the tap when needed. You'd want to wire up the pump to automatically cutoff when the top barrel is full.

My pump has a small outlet that is covered to capture the oil that may spray out if the pump is turned on without a load, or when first pumping down a system so the oil doesn't get blown out of the pump but is drained back into the pump. Most mechanical high vacuum pumps are rotary vane these use a fiber plate and the oil to make the seal. If there is no load the pump pulls more oil in and pushes it out the exhaust (some pumps do make big oil messes if not loaded for long or allowed full flow at the start of a pump down. Some less expensive pumps have a port to direct the oil into a container (vacuum pump oil can get expensive). There should be a warning if the pump doesn't have an oil reclaim system because it can be very messy.