Help clarify how the oil heads into the rocker arms...

Hi all!

New to the forum! I think this is the right spot to post a question regarding my 2015 Sierra 6.0 vortec? I have the L96 engine with VVT and I bought this truck for an absolute steal! The dealership had it ridiculously marked down due to them saying it needed a new engine. Their in house mechanic gave it a once over and said it had a tic. He noted it was not getting oil to the rocker arms and said it needed a new engine. I bough the truck with the plan to put a new engine in it but the engine runs great other than a very slight "squeek" and there being no oil getting to the top end. I've done a lot of work on LS engines in the past with past vehicles but never had to chase down a dry top problem. I'm a little confused about as to how the rockers are getting oil. Some places I read, people say the oil goes to the back of the block and then up oil galleries which feed the rockers, but then other websites mention the camshaft has a bad bearing which results in no oil to the rocker arms. I'm currently tearing down the engine now and will hopefully have the camshaft out by the weekend. The camshaft will hopefully shed some light on the issue as well as the possible squeeking. The squeeking noise matches the RPM of the camshaft, not the crank, fyi. Help me understand more about why a bad camshaft journal/bearing would cause the rockers to starve. Is it because a bad bearing/journal will "bleed" all the pressure out and not allow the top end to have oil pressure? Or does the oil somehow flow "through" these cam bearings and up into the rocker arms? Btw, all 16 pushrods are starving for oil. I pulled the pushrods out and they were not clogged. I also looked down into the lifters and they did not look gunked up by any means. The rockers do not appear stuck by any means and there is little to no play in the rocker arms. Other than pulling the camshaft, where should I start looking? Also, the oil pressure measured at the rear of the block shows normal. Thanks in advance!