It doesn't sound like the engine's running badly at all. The top RPM sounds good, and if the idle and transition are good, then it's running well.

As far as the exhaust stream is concerned, you should capture some of the oil on a piece of white card stock. Just hold the card near the muffler outlet and let a puddle of oil accumulate. Take the card away, and let the oil absorb into the card. If there are any metal particles in the exhaust stream, you'll see them on the card. If there are no particles, then the oil is most likely 'cooking' inside the muffler. That can turn a synthetic oil dark. Also, if the engine's running a bit hot, that can also cook the oil.

You may wish to try a fuel with a bit more oil content to see how it performes. The big 'Tigres need a fuel with about 15% oil content until the first few gallons' worth of fuel's been run through them. Then you can go to a low-oil fuel if it's mostly castor oil (that's per SuperTigre).