my 74 super does not mention multi-grade oils
Probably the top 3 true synthetics are Mobil 1 15-50; Pennzoil Platinum and Castrol 0-40 (this is the so-called German Castrol which is somewhat difficult to find, but meets some of the toughest standards out there.) I've used the Mobil Extended in an early 911 only because I got a free case - it's expensive stuff. As for the above mentioned fuel contamination discussion, if guys are running open crankcase vents on the street, then frequent oil changing is even that more important vs. long drain interval oils.