"If it really is a Mercruiser,
"If it really is a Mercruiser, you should be able to call any Mercruiser dealer and they can look it up. Their parts and service manuals are in a database called Midas and there's a serial number range for every motor/drive/transom ass'y they made. Since it's a mid-'90s motor. there should be a plastic cover on the flame arrestor and that will have all of the serial numbers and oil types, along with tune-up specs.
He may have omitted the prefix to the number. Have him read it to you. What kind of boat is it from? If he has the original hull number, the boat manufacturer can be contacted because they have to keep records of all boats made by them in case there's a safety recall."