Numbers such as 5W20 and 10W30 indicate the WEIGHT of the oil, which
dependent upon your area's ambient temperature:

20W40 and 20W50 are good from a low of no more than approximately 15
degrees F to a high of well over 100 degrees F.

10W40 is good from a low of approximately 0 degrees F to a high of well
over 100 degrees F.

10W30 is good from a low of approximately 0 degrees F to a high of no
more than approximately 90 degrees F.

5W30 is good from a low of way under -20 degrees F to a high of no more
than approximately 35 degrees F.

5W20 is good from a low of way under -20 degrees F to a high of no more
than approximately 15 degrees F (brrrrr).

The TYPE of oil recommended, however, is SF grade detergent oil, BUT...

Since you are asking about a 1988 car, I assume that you had recently
purchased it from someone else -- NOT that you had owned it yourself
since 1988 and just now got around to changing the oil. If the previous
owner had been using SYNTHETIC oil, then you must continue with
synthetic oil.