I run my oil for 10k of highway use too. I used to send samples to the lab when I was extending the intervals. The report always came back with everything looking good....

I still maintain that if the oil isn't good enough or clean enough for your $75 model airplane engine than it certainly wasn't clean enough for relatively epensive and important car and it should have been drained sooner.

I do think there is more carbon build up though than castor oil and it ceartainly is black!

On my .19's and down to .09's I use my BBB 1/2 a mix which is half commercial fuel so I have a much higher either content and those and it just makes the small engine run better and they don't use much fuel anyway.

On PAW .033 and down I use straight commercial model airplane fue because the like it and don't burn enough fuel to even cosider the cost.....