The gas they'll use is R134a which has a reputation as a leaky gas due to its molecular structure.
It replaced R12 (kills polar bears type gas) years ago but is not as efficient.
650g is the correct charge when weighed into the system.
The oil goes into the compressor and some is carried around with the refrigerant to lubricate the various seals etc which would leak if allowed to dry out.
The compressor clutch is an electro-magnetic device, a loud clunk signals its engagement and the centre of the disc within the belted wheel will spin...and hopefully some cold air will emerge from your vents.
Sprint - 33 - Sprint - 33 - 155 - GTV - GTV - 156 - GT ...gone German for now though.