Royal Purple is basic synthetic oil with purple food coloring for ppl who like paying $8/qt for oil...
M1 is good but also overkill IMO. You will be totally fine with Motorcraft and a Motorcraft filter which is what Ford puts in from the factory and warranties them on. You can get it for $17 per 5 quart jug at wal mart and the filter is like $4 at most. I use Motorcraft in all my vehicles or sometimes I use Super Tech (wal mart's synthetic) if its available in the weight I need.
Unless you are running your motor up to redline for prolonged periods or racing competitively, you do not need any more than a good basic blend or synthetic like MC or ST. And if you DO, for whatever reason, require a crazy omgwtfbbq oil, then you run Amsoil, not RP.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.