The original coolant was furnished by Texaco. It is a "conventional green" coolant. "Conventional green" is getting hard to find. It is high is Silacates and Phosphates. Probably the easiest to find that is a close match is Peak brand green coolant.
As to if it is the best to use or not, that opens up a whole big can of worms.
First of all, Ford has specified that their new gold colored coolant can be backfitted to all Ford products (except for the Mercury Villager, Cougar, and Electric Ranger) to include the 1999 year model, so according to Ford, you can use this fluid. It is a big improvement over conventional green. The supplier is Zerox (Valvoline) and it is sold under the Zerox label as G05. It has less yellow dye than what is in the Ford Motorcraft bottle, but it is otherwise identical. If you switch over, you should do a thorough flush first.
I won't get into General Motors Dexcool and it's clones (Prestone Dexcool and Prestone "all makes and all models") because I'm not in the mood to get on my soap box about both the good and bad of this product. Also, it would take a while to explain just what OAT (organic acid technology is all about). Ford believes that their Gold (Zerox G05) which is a HOAT (hybrid organic acid technology) is superior to Dexcool (OAT).
