i recently had problem on the oil pressure and have damaged the crank shaft on my 454 clock. i ve rebuild the engine by changing the crank shaft bearings, oil pump and the conrods.
after the job has been done i went for the sea trials which resulted to be in very well results like at idle oil pressure is 40PSI and on high RPM it goes up to 80PSI. after performance at idle RPM oil pressure drop at 40PSI. after 20hrs of running oil pressure is dropping to 20 and sometimes even less when i push it hard to 5 PSI. have you any idea what could be the issue?

Sounds like you're cooking your oil. Get an oil temp gauge before you fry another engine!

Kind of sounds like what I did once(when I was younger). I had the oil lines backwards so no oil cooling or filtering, spun a rod bearing. Expensive learning experience. I bought an oil temp gauge after that.

What he said , plus change the oil .

take some time and cut the oil filter open to see if you have debriss in the filter before you get running again to make sure you dont do any more damage if it's temp you may have an obstruction in cooler from an impeller failure or something of that type. Best of luck Laz Mesa

If you used the same oil cooler you may have released metal in your new rebuild.Theres no way you can remove all the trapped metal.All good advice giving.5#
You should see atleast 10PSI for every 1000 rpms.
Good Luck