The vehicle is a 1971 Ford Mustang Mach 1 with a 351 cleveland motor and a FMX transmission.What oil weight do I need for my 351 cleveland motor?
where do you live ?, If you live in a area of moderate temperature's 10w30 would work the best. If you live in a desert area with a lot of heat I would use 20w50. Depending on what your compression ratio, if the oil passage to the heads had ever been drilled and chambered, and the temperature- would make me wonder if I would support the use of 5w30. The 351 is not a low friction engine and 5w30 oil were not in common use or approved in 1971 for this engine.Because your summer temperatures go above 90 degrees I would swap half way threw the year. use 10w30 in Fall and Winter, and 20w50 in Spring and Summer. As for using a straight weight oil--that went out with the 1950's and the belief that a engine is worn out at 50,000 miles.What oil weight do I need for my 351 cleveland motor?
With a 351c engine I would run 30 weight without the 10w.
10w30 would be the best in your case.
Can't go wrong with 5W-30 or 10W-30, I personally run 10W-30 in my 302