I've had an '89 420 for several years and the most important thing I learned was the timing chain guide rails have to be replaces at about ten years or you lose the left head and possibly a piston like I did. Just got done putting a short block in to fix the repair the shop did with a used piston that didn't quite fill the hole. Look at this page to see the problem: http://www.import-car.com/ic/ic40024.htm
It shows what to look for. My car gets 22 mpg on the Interstate runs I do of close to 400 miles each way to parents or in-laws. I-80 has a cruising speed of anywhere from 72 to 82 depending on which state you're in and the car goes with the rest of them. My 2 cents.