It took me a few tries to get this working. You have to upgrade on exports and meet a certain $$ requirement.
What I did to get the upgrades was this... In my trading depot, I made the ALL oil holding facilities. I choose USE LOCALLY on them and waited until they were all totally full. Then, I made sure there was a big, clear road from the trade depot to the exit from my city boundary... The choose EXPORT.
The mass exodus of oil and incoming trade money helped me to hit the $$ goal for my next upgrade.