Your check engine light comes on to tell you to have the dealer do a PCM Quick Test. The PCM is the computer that monitors and drives all the systems. Sometimes it will, and sometimes it may not, display on the dealers equipment something called a DTC or Diagnostic Trouble Code. If your dealer checked it and said not to worry till Friday, then I wouldn't worry and go see them on Friday. It can be one of a hundred odd codes. They will check it, fix the problem, or if it is a code that came on for something like the wrong octane gas or some sensor wire that fell off, then they will simply reset the PCM, the light goes out, and you are on your way.
There is no way the computer knows if you changed your oil or not so don't worry about that suggestion.
Unless you have an after market chip or some heavy duty mods to your motor, you are wasting money and not doing your engine any favors by burning Premium gasoline. The Zetec is designed for 87 Octane, use it. Don't worry about whether you caused any problems by using it; you didn't. It may or may not have set off the light...only PCM test will tell the tech that.
Hope this answered your questions.
Bobby