Pressure relief valves are often underestimated, blocked or not replaced. Whenever fitting aftermarket oil cooler sandwich plates on your RB, did you ever realise you just eliminated your oil bypass valve/valves that sit in your factory oil cooler block? Yep, theres a blocked oil gallery in the block that goes unnoticed.
There are engine builders out there that recommend removing them and blocking the gallery, thats your call, but please understand the reason they are there, and Nissan spent a stack of money putting them there in the first place.
Blocking the relief valve forces all your oil to be run through your oil filter. If you think about it, there is no need to filter all your oil. If anything, it causes a major restriction, false oil pressure readings, premature ballooning and damage to oil filter element. At the end of the day, its another spun bearing in your RB, and thats far from something you want.

Oil filters
If your oil filter blocks for any reason, without an oil pressure relief valve you will lose oil pressure and thats game over.

So, when rebuilding an RB and cleaning out oil galleries after engine damage, how do you get the metal shavings stuck in the spring behind the baring valve?

How do you clean out the main feed oil gallery through the block?

Put simply, you remove all the bungs and relief valves and replace them.