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Thoughts on oil pressure. I searched the site but did not find an answer.

My car shows a strong 40 lbs when starting cold. I am running 10w-30w Rotilla (non synthetic) because of the additive pkg... recommended by my fav Packard mechanic.

When I am traveling around 60 mph in overdrive, the oil pressure is about 28 lbs. At idle, it drops to about 22 lbs.

I have the 356 engine.

Question:. Is this within spec?

Also, when this engine was built there was no multiple viscosity oil. Is it best to run it at 30W in warm weather?

The Engine Service Manual indicates normal oil pressure for the 356 engine to be 50 psi at road speeds.

Keep in mind that the car's build-in oil pressure gauge (if it has one) is not a precision instrument and is really little better than a low pressure light. It tells you that there is pressure but it not at all accurate as to how much pressure. For that you'll need some kind of add-on gauge or a test tool where you can see what it reads compared to the one in the car.

Lots of confusion and hearsay on oils, mostly due to marketing info.(opinion)
The rotella is formulated for diesel engines which have a higher soot load/cylinder pressure. There's no problem using it, so long as your in the visco range. Some are using the 15-40, which is not what you probably should. Mostly higher visco affects the early pickup of the oil pump when cold. Pump needs some viscosity to function but if oils stiff then it won't flow well to the inlet when it's needed most, at starting, increasing wear.
10-30 and 30 will be the same at the operating temperature. Otherwise, it won't say 30 in the second number.
They just add viscosity modifiers and test the oil at two temperatures to get the numbers. It's compared to the straight base oil numbers.(SAE/ASTM methods) There's not that much real difference as the numbers indicate. eg; 30 is not 3 x 10 here. (I have the curves.) It just the accepted method of yore.(tradition)
I personally don't like using the modern diesel additives, in an older engine that's not been rebuilt and possible sludged a tad.
Some sludge dislodges and plugs your passages and bad things will happen. You have to change frequently at first til no more crud. Oil service used to include annual pan cleanings.
You don't say what year or if it has partial or full filter or not, or condition/mileage, making ticking noise or not, etc. Covering the bases.

I would also verify the electric gauge reading with an independent direct reading mechanical gauge before accepting those numbers as completely accurate.

You can plumb and hide a permanent mechanical gauge in the engine compartment by adding a tee to the existing sender port or use a spare port off the main oil gallery on the passenger side of the engine. Both ports are fed directly off the pump and should read virtually the same.