Slow to build oil pressure

Hello,
I have had a problem on my 69 Valiant where if I let the car sit for more than 24 hours and I go to start it up. It does not build oil pressure, and even starts knocking for a second or two. Then all of the sudden the oil pressure builds, engine quiets down. I run at 45 psi at Idle and 55 while driving. Got 15-40 oil in it and a wix filter on it.


Any ideas what could cause this? I don’t want to ruin an engine that runs great otherwise.

Thanks in advance