You've got a good point that the oil flow through the turbo is really small and therefore difficult to measure. Better just to measure the CHRA temperature directly with a surface mount.
But where to get the thermocouple or RTD? And can't I just buy the RTD by itself and calibrate it by hand using an ohmmeter? It seems like I should be able to get my answer for less the $40. Doesn't have to be accurate, just precise and repeatable.