WOW...thats the ONLY time I have ever heard an engine builder recc'd STP....STP is basically a motor honey made to THICKEN
oil and stop it from going past worn rings...'stop smoke' is the primary function.
The modern oils are head and shoulders above ANYTHING the 60/70's motor foundries ever dreamed of. The newest line of GL spec oils have to meet aeration, anti-sludging and shear strength rules far in excess of a street motor requirement. This is why the mfgs can say it's OK to chane
oil every 5,000 miles instead of 3. Some even say 7,500 now. I run Royal Purple in my Cobras and my GT350, but the family cars get 5w30 Valvoline. I would say that ANY national brand
oil (Valvoline, Castrol, etc..) would be fine WITHOUT any additives. The STP may actually HARM the system in the long run.