I would guess (but who knows?) that the tech would have remarked if he felt your slides were worn to that degree, and even then I'd be skeptical that the oil was coming from your valves (unless you pour it onto them at every opportunity).
Some slide greases (or substances that people use for slide grease) do break down over time and degenerate into what appears to be a thin kind of oil. So my first question would be "What have you been using for slide grease?" And my second question would be "How often do you clean off your slide and reapply the grease?"
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)