As far as I know, each level of synthetic oil tech lets you build another refinery in each state.
So lvl 1 means you can build 1 in every state that still has a building slot free, lvl 2 means that you can now build a 2nd one in each state that already has one in it(assuming you still have building slots free).
edit -
This link: https://www.reddit.com/r/hoi4/comments/4mrskf/ive_played_100_hours_of_hoi4_here_are_some_tips/
Is where I found the refinery info, plus has a lot of other tips/answers to common questions.
Thanks! I was afraid of that, largely useless it seems (dumb for me to build as USA anyway, but I wanted domestic rubber). Very helpful tips, too.
"Battleplan system starts to make a lot of sense ~80 hours into the game" only 72 hours to go!