What would happen if I used medium octane gas in my premium required gas 2003 Acura MDX?

You can try using 87 octane if you like. Just fill the tank 1/3rd of the way when about empty. Then drive the car for 20 miles so it's at operating temperature. Find a long big incline and accelerate fast up it. Listen to the motor for pinging which sounds like a metallic popcorn noise. If you have pinging you need more octane in the gasoline. A motor that has pinging will have holes eaten in the pistons over time and ruin the motor.

THINGS are DIFFERENT NOW that we have EUROPEAN GRADE LOW SULFUR fuel out there and IRIDIUM spark plugs! The THING about DENSO twin tip IRIDIUM LASER PLUGS, is that an engine MADE from PREMIUM< may NOW use REGULAR and run as GOOD or BETTER! I tested on my older BENZ cars! T IT WORKS GREAT! The IRIDIUM reacts at the MOLECULAR level to give MORE POWER from from EACH droplet of FUEL! In a DODGE HEMI< it went from 14 MPG to 19.5 on the HIGHWAY! But the CITY driving did NOT improve very much! >>my BENZ got 32 EVERY DAY and SMOOTHE running! GOOD LUCK and BYE NOW! 

When I bought a used car a few years ago, I didn't get an owner's manual with the car, so I just used regular grade fuel (87 octane) for the first month or so, until I found out that the manufacturer recommended premium grade fuel. When I switched to premium grade fuel (92 octane), I got noticeably better fuel mileage AND better performance.

If your car is designed to run on Premium grade fuel, then THAT is what you should use!

less power that you would not notice. Run a FULL tank of premium gas on the freeway then FULL tank of medium. Record the milage. This will be your answer.

It is your car. You are the one who will be fixing it if the CEL turns on.