You know everyone's got a preference and there are a lot of misinformation stories about oil out there. The real truth is that any SAE approved oil meets or exceeds the manufactures specs. I personally prefer Valvoline 5 w 30. But as long as you change your oil regularly any SAE oil will work in your 5.0.
My ( 1991 ) Ford Mustang 5.0 LX came from the factory with ( 5W-30 )
The 2001 Ford Mustang has overhead valves (OHV).
The 2004 Ford Mustang has overhead valves (OHV).
The 2003 Ford Mustang has overhead valves (OHV).
The 2002 Ford Mustang has overhead valves (OHV).
The 2008 Ford Mustang has single overhead cam (SOHC).
The 2014 Ford Mustang has double overhead cam (DOHC).
The 2006 Ford Mustang has single overhead cam (SOHC).
The 2013 Ford Mustang has double overhead cam (DOHC).
The 2010 Ford Mustang has single overhead cam (SOHC).
The 2009 Ford Mustang has single overhead cam (SOHC).
The 2005 Ford Mustang has single overhead cam (SOHC).