Unleaded fuel only must be used in all gasoline models. Leaded fuel will damage the catalytic converter almost immediately. This will increase emissions critically, and may cause the catalyst material to break up and clog the exhaust system.
Use a fuel with an octane rating of 87 (R+M/2). This is an average of the two methods of rating octane-Research and Motor. If a fuel is rated by the Research method only, the required rating is 91.
It always pays to buy a reputable brand of fuel. It is best to purchase fuel from a busy station where a large volume is pumped every day, as this helps protect you from dirt and water in the fuel. If you encounter engine ping or run-on, you might want to try a different brand or a slightly higher octane rated grade of fuel. If the engine exhibits a chronic knock problem which cannot be readily cured by changing the fuel you're using, it is wise to check the ignition timing and reset it, if necessary. If the timing is correct and the engine still exhibits severe knock, there may be internal mechanical problems. Persistent knock is severely damaging to the engine and should be corrected.
If the engine runs on, and changing to a different fuel does not cure the problem, routine checks of ignition timing and idle speed should be made. Persistent run-on can be damaging to such engine parts as the timing chain or belt.
Mitsubishi recommends against the "indiscriminate use of fuel system cleaning agents." Occasional use of solvents added to the fuel tank to remove gum and varnish from the fuel system is permissible; however, continuous use or extremely frequent use can damage gasket and diaphragm parts used in the system.
Gasohol, a mixture of 10% ethanol (or grain alcohol) and 90% unleaded gasoline, may be used in your Mitsubishi. You should switch back to unleaded gasoline or try another brand of gasohol if you experience driveability problems. Some brands may contain special fuel additives designed to overcome certain types of problems that may occur with gasohol use.
DO NOT use gasoline containing methanol (wood alcohol), as they may damage the fuel system.