* Consult your owner's manual: This is the most reliable source. It will list the recommended oil weight for your specific engine (e.g., 289, 200, six-cylinder) and operating conditions.
* Common weights for the era: For most 1966 Mustang engines, 10W-30 was a common and generally suitable weight, particularly for moderate climates. In hotter climates, a higher viscosity like 20W-40 might be preferred. In colder climates, a lower viscosity like 5W-30 might be better for easier starting.
Without knowing the engine in your Mustang, it's impossible to give a definitive answer. Find your owner's manual or a reproduction of it online (many are available) to get the factory recommendation. If you can't find that, specifying the engine size (e.g., 289 cubic inch V8) will help narrow down the appropriate weight.
