It isn't just the viscosity of the oil that effects the oil pressure, it is the type.
It is not unusual at all to loose oil pressure as you put mileage on the oil.
This is VERY common with multiple viscosity oils since they are a blend of oil viscosity to begin with and have various additives which are affected by use, i.e., mileage, i.e., the ADDITIVES wear out. Some sooner then later.
10psi at idle hot to me is just indicating a stock oil pump.
There is an "old racer's adage" that says an engine needs 10psi of oil pressure for every 1,000 rpm.
The type of bearings installed internally in the engine also affect the pressure to some degree.
For instance some engine builders recommend 360° grooved main bearings while others will only groove one half of the bearing. That basically is because of the oil pressure variations like this.
I do remember MANY years ago engine builders recommending straight 30 weight oil if using 360° fully grooved main bearings.
Things have changed with improved "engineered" oils now.
Most engine builders that I know of now will recommend 360° grooved main bearings for a Cleveland.
I use them with a high volume oil pump. Some people don't like those because they put more strain on the distributors roll pins that retain the oil pump drive connection at the camshaft drive gear and the Clevelands in particular have shown a noticeable amount of roll pin failures with high volume oil pumps.
I will bet that if you change your oil to a known high performance oil and immediately look at the pressure, it will show higher? If that is so, then it's the oil itself.