Oil Pressure Question

[Lobo'74]

The engine in my '67 is a '79 350 that was rebuilt by the previous owner to the following specs:

*350, 4 bolt mains
*Bored .030 over
*Align Bored
*Parallel top of block
*New Cam bearings
*New freeze plugs
*New hypereutecic pistons, flat top
*Reconditioned connecting rods
*Crankshaft main and rod journals ground .010 under
*rotating assembly balanced
*Double roller timing chain
*Melling high volume, high pressure oil pump
*Speed Pro cam: Intake 224 deg duration, .450" Lift - Exhaust 224 deg duration, .460" lift
*New hydr lifters and push rods
*Sportsman II heads with 72cc chambers, 9.5 :1 compression, stainless valves 2.02 intake, 1.6 exhaust, heavy duty valve springs, screw in studs, guide plates
*Crane roller rocker arms (rollers on both fulcrum and tip)
*Edelbrock Performer RPM aluminum intake manifold
*Edelbrock #1470 750cfm 4 barrel carb
*HEI distrib
*Dyna-Max Headers

Here is my question... He said to run 20W-50 racing oil in it, so I'm using 20W-50 Casterol. Oil pressure on start up initally is above 80 psi. After driving it awhile it drops to say 60 psi when driving and 80 psi under moderately heavy acceleration. It is oh, say 40-60 at idle when warm.

The 80 and above seems high to me. Also, the 20W-50 seems like it will be too thick once summer is gone.

Is the pressure just a result of the high volume, high pressure oil pump, or the "heavy" oil? Would it be a bad thing to go to 10W-40 oil?

I'm new to high performance engines and don't want to screw up and make an expensive mistake by doing something stupid, or by not doing something I should.

As usual, thanks for any help and insights.