The ECU will vary the timing based on the fuel. It'll run whatever fine.
From the Focus St Supplemental manual:
Regular unleaded gasoline with a
pump (R+M)/2 octane rating of 87 is
recommended. Some stations offer
fuels posted as Regular with an
octane rating below 87, particularly
in high altitude areas. Fuels with octane levels below 87 are not
recommended. Premium fuel with an octane rating of 93 or higher will
provide improved performance and is recommended for severe duty or
high performance usage."
Meaning when the engine heats up all the way, your timing will vary to avoid potential detonation/pre-ignition when not running higher octanes