I went on the Mobil 1 web site and did the "help me decide" for a 1980 Plymouth Volare (3.7L 6 cylinder, 20,000 miles/year, out of warranty) and got the following results for "ultimate protection":
Quote:
What's the right oil for my car?
Our products in the viscosity right for your vehicle We've taken what you told us about your vehicle and driving conditions, and using an auto industry database, have determined our products in the viscosity recommended by your vehicle manufacturer. These products are shown categorized by the level of protection they provide. To learn more about a product, click its name.
Current Vehicle:
Year: 1980
Make: Plymouth
Model: Volare
Engine Type: 6cyl. 3.7Liter Naturally Aspirated
Ultimate Protection
Mobil 1 Extended Performance 10W-30
Mobil 1 Extended Performance 10W-30 fully synthetic motor oil is designed for today's longer service intervals. It provides guaranteed performance and protection for 15,000 miles or one year. Mobil 1 Extended Performance products with the Advanced SuperSyn System contain extra performance additives to help deliver outstanding engine protection.
Mobil 1 10W-30
A fully synthetic motor oil, Mobil 1 10W-30 with SuperSyn Technology, exceeds warranty requirements for gasoline engines where an API certified oil is specified. Mobil 1 is recommended by leading car manufacturers as initial fill.
Mobil 1 0W-30 Advanced Fuel Economy
For improved fuel economy, consider using Mobil 1 0W-30 Advanced Fuel Economy which is engineered to deliver outstanding engine protection and to offer improved fuel economy in vehicles where SAE 0W-30, 5W-30 or 10W-30 oil is recommended.
SM oils have lower ZDDP levels than SL oils but Mobil is still recommending Mobil 1 for slant sixes. For best wear protection and detergency, Widman (Corvair article) recommends CI-4 heavy duty diesel oils. A SAE 5W-40 diesel oil like Rotella would have similar viscosity to a SAE 30 oil.
My
1965 Valiant owner's manual recommends the following:
Single Grades
SAE 30 - When temperatures are consistently above 32°F.
SAE 10W - When temperature ranges average between 32°F and -10°F.
Multigrades
SAE 20W-40 When temperatures are consistently above 32°F.
SAE 10W-30 Suitable for year round for most parts of Canada, may be used as low as -10°F.
SAE 5W-20 Recommend if temperatures are consistently below -10°F.
You could switch to the 5W-40 diesel oil but your fuel economy would suffer slightly. The factory recommended SAE 30 or SAE 20W-40 for temperatures above 32°F so a 5W-40 would be a good choice as a summer-time oil. Unless you have extremely stiff valve springs, I think the wear additives in SM oils are sufficient for most slant six engines.
Does anyone know of any 0W-30 CI-4 diesel oils available in the USA?