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Seems like there have been discussions of this in the past on the forum. Most oils are petroleum based and this is a result of the interaction of the oil and any contaminants (water, dust, etc.) interacting with the oil. Ever look at that pretty engine oil AFTER it comes out of your car? Same kind of thing here. I have no experience with Ultra Pure oil but I know I don't get as much deposited since I switched to Hetman's oils and slide lubes.

i had that one my trumpet(when i was young and stupid) and i used to hae it on my tuba when i was using Blue Juice or Al Cass but since i switched to this stuff called Pro Oil Hybrid. it doesn't do that at all. actually when i pull my valves out they have nothing on them (except for the occasional lint piece) but here where i go to school we do have tha tproblem also with out sousaphones. but i think that may be because or the lack of oil on the valves.

Since it doesn't affect the horn in anyway then there isn't anything to worry about. I had it on my horn when it was new too. My teacher says it's something that they used to clean the inside of the horn with when it's new. I'm not very sure about that...but it could be some sort of a lapping compound.

Just wait until your next scheduled hypersonic cleaning with your tech, and everything should be alright. Make sure to tell him about it so he can proceed accordingly.