I never put anything but full synthetic oil in my cars. It cost a lot more but is much better for the engine. Viscosity would depend on the weather where you live/drive.
If your car is under warranty use the recommended oil as described in your owners manual. On the 03-04 SVT cobras if you had motor issues they pulled oil did a viscosity test and if it was not to spec your warranty was void. Don't know if this is still occurring but would not be surprised . Steele
