Yes a suction gun will work but my experience with both I would choose the screw on valve for the quart oil container. There's no hose to drip or clean up when finished. Each time you remove the hose to get another 1/2-qt refill, and that would be 12 times, again chance to drip and create additional messes. I'm a bit of a neat guy when it comes to this stuff and don't like the oil except in the engine. Either choice would work.
Yes, I make a mess on the floor quite a bit, always put towls down, or wipe/spay off customers vehicles if fluids get spilt on them. I work on commision, so I tell the boss I dont get payed to keep the place clean, there is someone making minumum wage for that- and really he doesn't care when I'm making him money!
Cheap easy way to add oil when needed or when I change my oil. I just use a 6 ft ladder with a paint tray shelf to set the oil container on and pump away. The neighbors look at me kinda strange but it works. I just store it in a zip lock bag in the shed.