Hi Dan, I am living in Spain (near Valencia). On my 302 with approx 400+Hp I am running Castrol Magnatek 10/40. No problem in winter or summer times. Driving the car be 10° up to 35° on sea side and mounten roads.
Oil pressure cold 50psi, warm 40psi and accelerating 60psi.
Thiner
oil has more flow and aswell les stress on the distributer gear, which is moving the
oil pump drive shaft.
Good oil prices and promt service.
Levoil -