A solution of filtered or distilled water and a modest percentage of isopropyl alcohol (substantially less than 50%) is safe to use on stubborn gunk. Some advisors recommend vinegar instead of isopropyl alcohol. Do not use any other type of alcohol or any other chemicals. Never apply liquid directly to the screen -- apply only to the microfiber cloth to moisten, but not dripping wet.
display used in the new "M" radios and the Maestro is the
same type used in smartphones,
tablets and laptops so cleaning is the same.
Two important things to remember is you must use the correct cleaning solution and the correct Non abrasive cloth made for screen cleaning or you will damage the screen.
links to an examples of cleaning products for screens.