A bit late to post but I have the same car.
I have used both Motul and Shell, with the Motul 300V Competition 15W50 you have an oil pressure of 4,5 to 5,5 with less range.
After a service at Ferrari, they change to Shell Helix Ultra 5W40 (advocated by Ferrari), and my pressure was from 1 (1000r/m) to 4,5/5 (high rev) which I consider as too low. I will definitely get back to the Motul next time.
I precise that my oil hoses are new.
You can also adjust the pressure around the pump but I don't do mecanich myself.
Hope it helps.
308 GTB 1979