For a normal service you just measure what comes out and replace it with the same amount. You physically can't drain all of it. It holds a good deal and the most you'll likely get out is about 5l if you let it drain. If it's too full now then put a bit less back in?
Have the engine hot after a run, put you foot on the brake and cycle through PRND21 slowly a few times, then into park and with the engine running, wipe the dipstick and check the level. You don't check it cold really.
Each time you swap the oil, only part of it comes out. You aren't swapping it, you are refreshing it.
Welcome to the club. Please don't forget to post an intro in the appropriate section. It's a club requirement. Nice looking 80.