Does anyone know whether the fuel pump is pressed once or held down ?

Just keep pressing and you can't be wrong! Some oil guns have a plunger, so you can take a shortcut.

Keep holding the fuel pump, and it will pop up automatically when it's full.

There's a buckle that needs to be pulled to be fixed, and it will automatically stop when full. You can ask the locals at the gas station.

I keep pressing it and it will automatically jump when full

Always, until the trigger is jumped.

Always.

I'm sorry, but I'm not sure what you're asking. Could you please provide more context or clarify your question?

Always on full will trigger a jump