It is what the cap says but they are a lot of factors that come into play. Think about this: 99-00 mustangs required 5w-30 and then suddenly in 01+ they changed to 5w-20. Was there any change at all inside the engine other than CAFE standards coming into play?
Also some relevant quotes:
Mike Riley, Product Design Engineer with Ford Motor Company:
Quote:
"Moving to 5w20 was driven by ... CAFE requirements. The company believes the switch to lighter viscosity 5w20 oils will reduce carbon dioxide emissions by 190,000 metric tons a year and reduce US fuel consumption by over 21,000,000 gallons a year."
FRPP Group:
Quote:
"DO NOT go back to using 5w-20"
Scott Whitehead - Engine Development Systems Engineer, Ford Motor Company:
Quote:
"I've seen the dyno testing. I've probably looked at several hundred engines disassembled and spread out on inspection tables.
And I've been involved in testing to resolve issues where varying the oil viscosity was part of the test. Please do not put 5w-20 or less into a mod motor. Please. This is especially true in 4v motors. 5W-30 is probably a good oil for the street with mixed temperatures."
Unless I see anyone with more pedigree stating for 5w-20 then I will go with what the dude that designed the 3v says...
http://www.svtperformance.com/forums/showthread.php?675202-Oil-Viscosity-for-DOHC