I've been running Amsoil synthetic since I purchased my Stang over 5 years ago. Put it in at 21,500 and have had very little "residue" oil that appears to be coming from the valve cover gaskets. Again though, it is very minor - I just clean off the covers and polish them a couple times a year. Amsoil = Good stuff.
E
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1991 5.0 LX Coupe - 38,000 miles
13.17 @ 106.14 mph w/ 2.138 60'