Do you have a filthy, painted-over, but otherwise salvageable VIN, engine, transmission or other identification plate that is original to the car? Probably so, and you don't know how to clean it up without losing the very imprints you're trying to save.
Many of these plates were silk-screened or printed in easy-to-remove, oil-based inks, so you don't want to attempt a cleaning using solvents or abrasives. What should you do?
Do what the restoration people at museums do, of course! Use gel-type hand cleaners!
That's right, the art historians who regularly restore priceless paintings use everyday gel hand cleaners (you know, the greasy stuff that doesn't contain pumice!) to gently lift off layers of crud from the delicate finishes.
Use very soft cloth or cotton balls spread with a thin film of cleaner. Gently wipe the plate surface back and forth, changing the cleaning material as it picks up dirt and grease. Be patient, because this will take considerable time. Eventually your plate will look quite presentable, if not like new. When finished you can apply a little polish to the plate to brighten it up.
Your identification plate might not have anywhere near the value of a museum painting, but it's just as important to you, so clean it carefully.