* Condition of the engine: If the engine is in excellent condition and you maintain it meticulously, a 20W-50 might suffice. But if there's significant wear, a thicker oil like 25W-60 might be better to help manage clearances.
* Climate: In hotter climates, the higher viscosity of a 20W-50 or 25W-60 will offer better protection. In colder climates, a 10W-40 might be more suitable for easier starting, although a 20W-50 would work too, it just may make it harder to start in very cold weather.
* Modern vs. Conventional: While 20W-50 conventional is period-correct, modern formulations of 10W-40 (or even a 15W-40) offer better protection against wear and deposit formation. Many people successfully use high-quality modern oils in older engines. However, you should always check the API certifications to ensure compatibility.
* Additives: Avoid oils with high levels of zinc and phosphorus, as they can damage catalytic converters (even if you don't have one on a 1966 car, it's a good habit to get into)
Recommendation: Consult your Pontiac's owner's manual for the manufacturer's original recommendation. If you cannot find the manual, a good starting point would be a high-quality 20W-50 conventional oil or a modern 10W-40 meeting current API service specifications (check the oil bottle for the markings). You can then monitor your oil pressure and adjust viscosity as needed. A mechanic specializing in classic cars can also provide valuable advice tailored to your specific engine's condition.
