What pressure? The same cold tells me you may be reaching by pass pressure in the pump... 15 versus 0 should be a big difference cold
What my experiments with the two different oils had taught me is that these engines, when set up on the tight end of the Ford desired specifications, are NOT as oil pressure sensitive to different oil viscosities as once believed. Now, it you set up the engine using the loose end of the Ford DESIRED specification, like using a clearance of 0.0015" (50% + looser than my engines) instead of the 0.0005" to 0.0001" as I did, then a different in viscosity might translate to a different outcome than what I experienced.
I used a Melling high volume pump that has an adjustable pressure relief valve. I set mine to not open until the oil pressure was in excess of 100 psi. I wanted to be sure no oil was bypassed with an open pressure relief valve unless there was a catastrophic pressure build up.
Cold oil pressure with the 15w-50 at idle was always 80-85 psi, regardless of ambient air temperature. In my climate the highs were in the 100-110 F. range and the lows that I would drive in were 25 F or higher. It does get below 25 F in NE Oklahoma, but I can always find something else to do besides driving until it warms up a little.
I never drove the car until the water temperature got up to 180 and after the engine was at 180 for a few minutes, the oil pressure drops to the 45-65 level at idle. At that point I gave it a good exercise as often as possible, using the entire rpm range. At my self imposed red line of 6,500 rpm, my oil pressure was 70-80 psi, and by design well below the set opening of the oil pump pressure relief valve. When driven hard the engine temperature would vary from 190 to 205 depending on whether I was using the A/C I had added, and dependent on the outside air temperature. Those engine temperatures were exactly what I think is optimal for any SBF. Even 210-220 would not concern me in the slightest.
My vintage cars were essentially my daily drivers and most years they were driven 350 + days of the year.
One winter I decided to try Mobil 1 0w-40 as an experiment. You could've knocked me over with a feather when the oil pressure was essentially unchanged from what it was with the Mobil 1 15w-50.
The only other part of the oiling system besides the oil pump that was non-stock was a chrome moly oil pump drive shaft, and a set of Comp-Cams edm oiling lifters, They have a 0.012" in the lifter face right were it rubs on the cam. This puts pressurized oil right on the cam lobe / lifter juncture. 16 lifters, all with a 0.012" hole feeding pressurized oil to this critical area seems to have no impact on the overall oil pressure. Again, this was surprising to me. I had expected a drop in oil pressure from the addition of these lifters, but that didn't happen either. I wholeheartedly recommend theses lifters to anyone not using hydraulic lifters. They are great insurance again ever wiping a cam lobe.