Possibly you could wrap pipe heaters around a section of the pipe at the
beginning and look for a temperature increase at the other end of the pipe.

The oil would lose most of its heat as it travelled, but there might be a
temperature increased of a degree or so at the end if the oil was heated
enough at the beginning.

This could be detected with a thermistor in a Wheatsone bridge, amplified
by an opamp and fed to a PIC ADC input (what else?). Even a fraction of a
degree temperature increase could easily be detected.

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