Why?
Change it now please and let us know what that does for your motor
I change all my newly rebuilt motors oil and filter after 20 to 40 minutes running times depending on the cam and lifter types, if it has a roller cams and lifter I change the oil and filter after 30+minutes, flat tappet type cam and lifters max of 30 minutes before changing both.
I cu the filter cover, outer shell, off so I can inspect the filter element after cutting all of it off the inner canister holder so I can inspect what is in the pleats

I always heard about the 1000 mile rule and followed it.



You are almost there so changing it won't hurt.





So where is the pvc connected to after the EFI And you said the pvc valve works but what about blow by? Take off the pvc valve and see how much "blow by" comes out after you are at operating temp. If it's excessive that's what is blowing oil into the intake.

This, and I also change the oil again at 500 miles. May be overkill, but I sleep better knowing for certain what is going on inside after cutting open the filter.

If you didn't have a problem before you replaced the intake manifold, and you have a problem now, go back to the last change, the intake. I'd replace the intake gaskets/ valley pan, and check the head surfaces on your new intake to be sure they are flat and not gouged or something. Did you reuse the valley pan, or install a new valley pan? Oil fouled plugs would be expected if the intake ports are not sealed off from the valley and is sucking oil into the intake, and low intake vacuum would be expected if the intake isn't sealed to the heads. Both of which would indicate poor gasket seal.

Oil on the headers would tell me your valve cover gaskets are leaking, or the front and/or rear corners of the valley pan are leaking.

You pulled the intake, change the oil after you replace the intake/valley pan gasket, a bit of antifreeze could have gotten into the motor with the intake manifold/gasket change. Change it again in another 500 miles, just in case.

Years ago I bought a rebuilt 440 pretty cheap, it was using a quart of oil every 500 miles. I changed the intake valley pan gasket and solved that problem. It was a great motor. The guy didn't believe me.

New aftermarket valve covers allowing the PCV to suck oil (no baffles)?

Check the inside of the PCV hose. Or the port its plugged into for oil.

If you pull the intake you should see signs of oil being sucked in, but all 8 cylinders, well either you forgot intake gaskets altogether or the intake is seriously mis-machined.

My engine is no longer blowing blue smoke. I've taken it for a few drives now. It has plenty of power. I am really stumped on what the issue was.

Strange? Maybe the lower intake was leaking a sealed itself after a few heat cycles

Must have been assembly or fabrication lube some where.