Von - are you just wanting romanian oil, not needing it? Cus we really don't understand how you could have gotten into a situation where you actually need it. Also, I believe nations will share their strategic resource with you if they are 'in your corner', even without being part of your faction. I don't remember how close they need to be do it, but I think they share it even before you can invite them.
Another idea I just had - Romania will cancel trades with anyone they're at war with. If they're in the axis, you can call them to arms against the allies, and unless they have some heavy trading to neutrals (I can't think of why they would), that could free up most of their oil and feel less gamey.
Also, from your description of canceling their trades, do you know about the console and the 'tag' command? You can pause the game, press the ` key, and a console will open. type 'tag XXX' where XXX is the country code, and you switch control to that country. You can do this without pausing, but if any time elapses, your entire country will be under AI control. Can lead to unexpected changes (not the least of which is that your army will be on AI control even after you switch back, with orders to go who knows where and do who knows what). The country codes for Germany and Romania are GER and ROM.