OP, this thread has accumulated some truths, some misguided intentions, and some whining. I'll only address the first two. Subaru H6s AFAIK all have the oil cooler. High copper by itself is not a concern on Subarus; copper in a UOA without lead or tin does not point to mechanical issues, it points to the cooler. Owner's manual says you can use conventional or synthetic; considering 99% of the engine is essentially identical to what was intially designed all the way back in 1999, there are no crazy oil requirements. In all reality (but not suggesting you should) there would likely be no issues running an API SL oil from back in 1999. Since it's not a turbo EJ, this engine design is not particularly demanding of oil. PUP is a great choice, especially the older stuff it appears you have. I would use every bit of it, and change it at the 7.5k intervals in the OM. As long as the engine and oil is up to temp on a regular basis, there's no real need to follow severe service.