Hey, looking for some advice on a 454 that I recently installed. After sitting for a while, all the oil is draining back into the pan and the pump is losing its prime. It takes around 30 seconds for oil pressure to build if I crank it. Could this be an issue with the oil filter or the bypass valve or do I need to pull the motor and check the pump?

When the engine is cold, the oil pressure is great, but once it warms up, I'm not getting much more than 20 psi at 2000 rpm. The oil pressure issue may be attributed to me using the wrong, lower-weight oil. I'm switching to a single weight oil but wanted to provide the info in case it is relevant. Thanks for any help!

If the filter is the type that is installed upside down, you need a filter with a built in anti-drain back valve.
Mobil1 M1-302 has one as well as many others.

The filter is in the standard orientation with the mouth facing upward. I pulled the filter off last night and it was full . There was also oil above the filter that leaked out when I pulled it.

what weight oil are you running,what is the oil pan capacity and what are the main&rod bearing clearances?

I was running 10w40 but will be switching to 50w . It's a stock oil pan. I think 8qts? I do not know the clearances. The motor was purchased rebuilt from Atlantic Marine here in FL.

Ayuh,..... The oil pump is in the base,.... it can't loose it's prime,.....

I suggest ya change the oil, 'n test it,.....

Btw,..... 10 lbs. per 1000 rpms is considered acceptable oil pressure,.....
'n Every motor starts out with higher oil pressure, which drops as the oil heats, 'n thins,.....

Yeah, I'm more worried about the lack of oil pressure at startup. Running the motor for 30-45 seconds without any oil pressure sounds dangerous.

Attach another pressure gauge to the motor to verify the no pressure at startup.