71 340 engine.Started on a run stand and oil pressure at 2000-2500 rpm was 100 lbs.Engine totally rebuilt,all bearings within specs.Oil used was Pennzoil 20-50.If I went with 5-30 would that bring the pressure down? Thanks in advance

Give it some break in time the pressure will go down some as things seat.

I'd be using a 10W-30. Yes, lower viscosity should produce less pressure & quicker flow. 100PSI is a lot, not necessary.

10w-40 or 10w-30 is what you want.

I also would suggest you start with 10W30. With flat tappet cam it is really important to use a high zinc oil or add zinc.

The 20W50 likely is already formulated with higher zinc as a classic car performance lube. Most 10W30 sold does not have enough.

Tell us what brand and type, stock, high pressure or hi volume,oil pump you used in your motor?
100 Lbs. is excessive in any SB, I've seen this happen in the past and ended up switching the pump to a standard 318 non high volume pump to get the pressure down to 65 to 70 lbs cold and 30 lbs. hot idling

The reason I say break it in is because I have had more than one new engine start with high oil pressure and after a few run in sessions it dropped to a normal level. I really don't think you have a problem if it has not been run long.

Go to napa and get 10w30 Valvoline VR1. 20w 50 is to thick unless you have large clearance

I agree. I have had several engines rebuilt and end up putting the viscosity in the engine that the owners manual calls for. By using the Valvoline VR 1 you have a high Zinc content oil which is what you want as others have said.

I agree, 10w30. I would go with a synthetic hi-zinc muscle car/race car formulation once the break-in is completed.

BTW how high on pressure does your gauge read ?
I've seen several new high volume pumps for SB LA motors have BB bypass valves in them which made the pump have over 140 Lbs. pressure while running above 1200 RPM with cold oil
The BB bypass valves have no holes in the bodies, the SB LA motor should have three or more holes in the sides of them
I had to drill a hole in the plug on the bypass passage to get the valves out and then either weld that hole up or use a new welsh cup after replacing the valve with the proper one, problems solved

On the subject of SB Oil Pumps. Is this a stuck open bypass valve?
This pump I thinks was new or had very little use. I was going to install it in my 340, but decided to take a peek inside first. Maybe the relief spring is broken?

That is one of the holes in the bypass valve to allow the oil to flow through it once it is forced open by the pump pressure, your golden
The BB valves have no holes in them

Had a 340 rebuilt about a year ago. The machine shop was very adamant about using Brad Penn break in oil for the first 500 miles. Keep your foot out of it and vary the RPMs as much as possible.

It really sounds like your machine shop is really old school on clearances and parts
You really need to load the rings as soon as possible to help seat them, DO NOT baby the motor for that long

Agreed, work that engine from the start, not too much RPM though.

The issue is that the valve is open with no oil pressure. The relief spring should have pushed the valve close. If I ran this pump I would then most likely have low oil pressure.

Take the pressed in plug out and examine the insides so you can see how that bypass valve works
The valve has to be shoved past it seat to make the holes in the valve allow oil to bypass into the other part of the pump
Let us know what you do and find out

When I get the chance I will do that. I first need to find where I put that pump.
Question.. I know they were standard pressure and high pressure relief springs, but are the relief springs interchangeable between the SB and BB pumps?

Speedway Motors has Joe Gibbs Driven Oil on sale right now. Hard to beat this quality oil for your 340 engine. HR-2: 10W-30, or HR-5: 10W-40.

https://www.speedwaymotors.com/Driven-Racing-Oil-02007-HR2-Conventional-10W30-Motor-Oil-12-Quarts,272680.html

http://www.drivenracingoil.com/