New optimizers all have "506" cast in them like the last gm production blocks. They will have the international double diamond in the valley. They also have different casting numbers than the gm blocks. Here's a picture of my optimizers lifter valley and the international markings:
Any gm block that's been magnafluxed and found to be crack free is a decent rebuilder. If it's not cracked by now it's a reasonably stable casting. However, be aware that they're almost all cracked, even the "fabled" 599 block. Mine was....
What does "oil from everywhere" mean?
Leaking from crank seals, valve cover, etc or was it blowing liquid oil out the filler tube?
Oil seeping everywhere is just an old tired engine. Spraying oil out the filler tube is a collapsed piston crown ( I've experienced that too), fragged rings or a scored cylinder bore.
The crankcase valve is properly termed am crankcase depression regulator (CDR). Unlike a pcv valve, it limits the vacuum that the engine pulls on the crankcase.
Now, reality check; if you've got lots of blowby in the crankcase and a high mileage engine, it's probably the rings and not the cdr.
Google "water test" and "6.5 diesel" to get a simple check you can perform to give you an idea of how bad it is ( or isn't).
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