You can always request a Used Oil Analysis to be sure. I once had the same color come from my 4 cylinder air cooled generator. it scared the crap outta me thinking it was bearing material but it wasn't. Still never identified it but it went away.

http://www.blackstone-labs.com/

 

There have been some discussions about what we put in out primary's. I am not a fan of running the same oil in all three places. Your description sounds like normal wear.