Useful handbook for first-time car owners; Your First Car, by George and Suzanne Fremon. Cockeysville, Md.: Liberty Publishing Company, Inc. (50 Scott Adam Rd., 21030), $3.95.

Do you really understand the difference between grease and oil? Do you know that a fuel pump should be replaced every 50,000 miles even though it may still have some life in it? The first bit of information may mostly help you to converse in the often-foreign language of your automobile mechanic. But the second piece of wisdom can save you from the all-too-frustrating position stop the soft shoulder, snarling at your misbehaving machinery.

George and Suzanne Fremon -- owners of nine cars betweeen them, five of which are still in operation -- feel that a little of both kinds of knowledge can rescue a car owner from prematurely sinking money into a replacement auto. With systematic care and a reliable mechanic who honors your requests, the Fremons feel that your car can carry on quite well, even into its teens.

"Your First Car" is a clear, simple, and intelligent approach toward protecting a large investment. It doesn't pretend to be a technical manual. But the reader of these pages will be well grounded in the basics of preventive maintenance: knowing what the automobile demands from you and what you should dema nd from your mechanic.