Perhaps you had some minor corrosion in your horn, and the new oil's anti-corrosion additives are ... er ... anti-corroding it -- resulting in the brown color as a suspension in the oil. If so, a thorough cleaning might change things. Or the color may disappear slowly over time as whatever is being dissolved by the oil is thoroughly removed from the instrument.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)