Ford began shifting their OHC engines to run on unleaded fuel in the late-1980s. Sometimes, they have M, MM, N, or NN stamped on the head at number four spark plug to identify this. With lead removed from fuel, older engines suffer from valve seat recession. If you can't contact the previous owner or have no way of checking, the question you need to ask yourself is how many miles you plan to do in the car? If you're only planning to do 2000-3000 miles a year, then simply running an additive in the fuel will suffice and protect against the damage to the valve seats. If you choose an additive that has an octane boost, you shouldn't suffer any negative effects on ignition timing (or if you do, it will only require minimal adjustment).