What kind of gas does the 2009 Acura MDX take?

The 2009 Acura MDX recommends the use of premium unleaded gasoline, with an octane of 91 or higher. Some drivers report success with 87 oR89 octane fuel, so it's not strictly prohibited -- just recommended against. Your engine was designed with 91 in mind, so using a lower octane fuel can lead to issues with engine knock; this can lead to expensive problems down the road, so it's likely better to shell out the few additional dollars per fill-up to avoid those huge costs. Lower octane fuel also has a lower flash point, so it may combust prematurely in the '09 MDX's engine. Over time, this can lead to severe knocking and damage. One or two low octane fuel tanks full likely won't kill You, but You shouldn't make a habit of it.