Had some experience with Royal Purple API 10w40 oil, ran it for 11000km in my rb26 on a single shiite fram filter (noob mistake) and the oil was still light to mid brown, admittedly the motor was fresh but the oil was still relatively clean and the pressure was still 3.2 bar at warm idle, 4 bar cold. No oil analysis just my personal observation for what its worth. Currently running RP HPS 10w40 with a K&N gold HP2008 oil filter, car seems very happy.