5w20 is a bad idea even for a naturally aspirated / stock compression Mustang, but its recommended by Ford for gas mileage. It does not give the best longevity to engines. We have found over the years, along with other engine experts on the 3v engines have all attested to 10w30 having the best wear characteristics. Like many PJ's, rings have a tendency to be an issue along with factory oil pumps.
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