Realize, however, and I was forgetting, that the 04/05 had an external PCV system where the crankcase vapours (from blow-by) are routed through an external tube about 1" in diameter that runs up to right-hand side of valve cover. The outlet for the crankcase is just below where the starter motor is and the the external tube then runs up to the valve cover (right hand side). You can check part numbers for valve covers and find they don't interchange either for this very reason. The 06/08 engines don't have this (external tube) as I believe the crankcase vapours are integral to the block and cylinder head. That is, the external tube is gone (as found on 04/05 engines) as it is cast into the head and block of the 06 thru 08 engines.

Long story short you may have blocked off your PCV system through a head/ block mismatch.... I compared two engines side by side 2008 vs 2005 which is why I know this. If this is the case the engine can run fine but pressures may build up in crankcase and this will, at some point, sooner or later cause seals to blow out. Weakest ones first I guess, could be oil pan seal or rear main seal, or.... anyways you'll end up with oil leaks.

Those were my observations anyways and why I concluded not interchangeable...