You need the correct spec oil, whether it is "synthetic" or not. If it is compatible with Dextron II / Mercon, or whatever the owner's manual says, then you are fine. The Chilton's book should be correct, those are pretty good, but if it isn't, someone here will know.
"Synthetic" is just a difference in how the oil was refined / processed. The spec (Dextron II / Mercon) will ensure that it is the correct viscosity, with the correct additives.
Just got to find something that is backward-compatible to 1988 BII transmissions.... do not obsess over "synthetic" vs. "mineral" because they ALL start out from the same crude oil.
Ford doesn't "make" oil anyway, they buy it from a refinery made to their specs. That's the key. Made to the correct specs.