Hi gang. 17MKZ3.0TT offered good information above. However, just to avoid confusion: Our MKZ engines are designed to run on fuel with a minimum octane of 87, and containing up to 15% ethanol (E15). They will get full advertised power using 93 octane, but will only suffer a loss of ~ 20-25 horsepower using 87 octane. And using 89-91 octane mid-grades will offer correspondingly lower ratio losses of horsepower than 87 octane. Full gas, non-ethanol fuels can be hard to find in many parts of the country, so have no worries about using E-10 fuels, or up to E-15, if it is all that is available.
More important than octane (other than minimum 87) is to use good quality fuels. So try to only use Top Tier fuels from name brand stations, avoiding cheap, generic gas stations whenever possible.
Essentially, if you really need full advertised power, use Top Tier, 93 octane fuels. If not, and you can get away with 375-380 horsepower in your daily drives, use 87 octane, save money and rest easy. Personally I use Top Tier, 87 octane fuel 95%+ of the time, unless I'm feeling a bit sporty. But then again, I do not drive my MKZ hard at all.
All of the above being said, I would still recommend everyone read their Owners Manual section concerning correct fuels to use. Just look in the Index under "Fuel Quality" for the correct pages in your copy.
Feel free to ask any questions, and good luck.