The type of oil a 1999 Ford F-150 takes depends on the engine it has. Ford F-150s from that year used several different engines. To know for sure, you need to
check your owner's manual. It will specify the correct oil type (e.g., 5W-30, 10W-30) and the API certification (e.g., SL, SJ).
Without knowing the engine, I can only give general advice: Most likely, it will require a conventional or synthetic blend motor oil in a 5W-30 or 10W-30 weight. However, always consult your owner's manual for the definitive answer. Using the wrong oil can damage your engine.
