I fought this issue for quite a while. I did all of the above mentioned tips and tricks. Nothing fixed it. I even plugged the old oil pickup, filled it with RTV and made a custom gasket. Didnt help. I ended up replacing the pickup line, and all the fittings including the one into the pan.

That finally solved the problem. Still dont know if it was a bad fitting, a leaky hose, or something with the pickup????? All i know is now itll sit for days and have oil pressure right away.