I have a 1988 420 SEL that quit moving forward one day right in the middle of heavy traffic near DC. I had the same thing of reverse but not much forward. I got it into a parking area and found a dry dipstick. I put in two quarts and it has run fine since. That was about a year and a half ago. Since then I put in part of a quart to bring it up. Also that was about 4 thousand miles back. Maybe I was lucky. Jerry