The 1988 Ford F-150 with a manual 5-speed transmission likely used
SAE 80W-90 gear oil. However, it's
crucial to check your owner's manual for the *exact* specification recommended by Ford for your specific truck. The manual will tell you the correct type and viscosity of gear oil.
Using the wrong fluid can severely damage your transmission. Don't rely solely on general advice; consult your owner's manual.
