Have a recently rebuilt 360. at first the oil pressure was 65 cold and about 40 hot now with a couple thousand miles on her i get 50 cold and 20 hot at about 2k rpms. it drops too 10 at idle hot.
I did notice that if I let off the gas at speed the pressure climbs to about 30 then drops back down as the rpm's drop. I'm going to put a hyvolume pump in it just cus I have a new one but not sure how much it will help. any ideas why the pressure is so low. Havent found any metal in the oil and no knocks in the bottom end.

Is there any smell of gas in the oil? If its rich and getting gas in the oil that would drop the pressure. If so changing the oil would bring it back up for a while.

...If it's not running rich and you smell gas in the oil, you might check the fuel pump diaphragm for a leak. If there's no gas smell, you might change to a thicker straight weight oil like 30,40, or 50 weight. I wouldn't use a hi-volume pump without a hi-volume pan.

Its in a truck so has a 6 qt pan. running 15-40 deisel oil in it. no smell of gas. had low pressure when oil was fresh. If there are excesive clearances in the motor causeing the low pressure a hi-volume pump would help some and the oil would likely drain back into the pan plenty fast as long as the problem is not in the heads. At least thats what I think considering my limited knowledge of internal workings.

you are correct in thinking that

rule of thumb is 10 lbs for every 1000 rpms

i have run less than 10 lbs at idle/20-25 lbs at 3500 rpms for yrs on some used sb

never had any trouble

when the lifters start to rattle then the oil psi is too low IMHO

my daily driver has over 200,000 miles on it all stock and was a cop engine to boot

idles at 750-850 rpms and has 20 lbs and 50-60 at anything above 1000 rpms

I would run it myself

Use the high volume pump - you will never pump out the pan unless you hold it at about 7 grand for an extended period of time and then I doubt it would happen. Runamny of them over the years and NEVER had a problem.

There is a pipe plug inside SB's that, if left out after cleaning, will cause low oil pressure. One of the SB guys can provide more details. That's worth checking.

Also, I've seen engines built with excessive bearing clearance start up with OK oil pressure then go down hill within 1000 miles. I think that's the next thing to check. The HV pump may help a bit but if the bearings are hurting it may not help for long.

Well got the pan off and no evidence of major engine problems ie: metal shavings .. so gona put the new pump on and hope for the best. I have had the plug missing problem on a diffrent motor and it had basicly no oil pressure right on startup. Also had one that they left the "pipe plug" out by the dist drive gear and that one had very low pressure also. I have verified that these plugs are in place. got to get a pan gasket new oil and filter and it will go back togather.

Well got it all back togater and not a significant improvement about 10 psi across the board. Rebuilt the quadrajet while I was at it runs a little better now. About ready to pass this down to the next owner not really to practical as a daily driver.