Originally Posted by upshift

I've heard of that controversy and have done my research. The older modular motors called for 5w30. This was before 5w20 was commonplace. The simple answer is that the motor was designed for thin, 20 weight oil, and thicker oil actually accelerates engine wear in that motor. I've talked to Ford engineers, city fleet managers, and have owned probably 15 Ford modular v8s. I've heard personally of two oil related failures: chain guides worn out after running 15w40 for 50,000 miles and 75k miles - separate cars.