The oil dipstick on a 1991 Ford F-150 is located on the
top of the engine, near the front. It's usually a yellow or bright colored handle that's easy to spot once you pop the hood. Look for a handle with a ring at the end. It's often near other fluid dipsticks (like the transmission dipstick if it's equipped with one).
However, the exact placement can vary slightly depending on the engine type (e.g., V6 vs. V8). To be absolutely sure, consult your owner's manual.
