Steve, I have increased the oil content to 9%. When I see black draining out of the timing chest in with the oil, there is a problem. At this oil concentration the oil drains out crystal clear. The 7.5% nitro sure keeps the engine running very well at all throttle settings. I was flying my corsair one day and the engine was running at peak tune and thus full power so it really unloaded in a dive..... kind of scary fast (unrealistically). That was when I had the blackish oil and running 8%. I of course richened the engine a bit. I, at Toledo bought 2 cases of 15% nitro 18% oil (instead if 16%) to dilute to 1 to 1 for the 9% (Wildcat brand with the small amount of castor blended in with the synthetic). The additional oil had no effect on the running qualities. I bet 10% would not either if a bit of nitro is present. -Tom