I had one of these cars, new, off the showroom floor. It was a Classic 660. I used Castrol 20W-50 from the day I got it until the day I sold it. I put well over 100,000 miles on it.
If it were mine, I would use, (1)Mobil 1 15W-50, (2)Mobil 1 10W-40 HM, (3)Valvoline 10W-40 MaxLife