Welcome to JA!
Here is a breakdown of what repair history is on 2003 c-12
Broken head bolt
cracked head gasket
prosody pitting on the cylinder sleeves
cracked o rings on the cylinder sleeves
cracked o ring on the spacer plate
cracked spacer plate
watter pump fount seal
These are all repairs we have seen here at this shop,
you can have one or more of the above problom
Let me know if i cab help more!
Here is what works for me
we have antifreeze in the oil so
we should go ahead and put
mains / rods/ thrust plate/ oil pump in
so what i do is
pull everything to the injectors on top
when you pull the insecters lay teem out in order
they will tell you if there us antifreeze passing by them by the color of the tips
if you have one that is all carbine up real bad thats the one to look at
be careful with the injectors the core is 300 bucks if that look damaged cat will charge you!
now pull the pan
and install your presser tester
pump it up to 15 psi let it sit
if we hear a leak but cant find it
a in less cost alternative to costly smoke checkers and dye kits is green food dye and watter,
safer then antifreeze and wont break the bank to test.
i would for the 20 bucks in gaskets and o rings pull the oil cooler presser test that off( after you test the system)