Welcome to the OEM forum, Phil.
My instinct would be to install a flow meter with a pulse output in the oil pipe line. The emonPi has a pulse input which you can use to count pulses to give the energy consumed, or by looking at the pulse rate you can deduce the power input to the boiler.
I don’t have experience of oil flow meters, so it’s a case of seeing what fits your pipe and your flow rate (i.e. you want a few pulses per second at the maximum normal flow rate if you want to see the power/energy consumption in near real time, but if you’re only interested in hourly or daily totals, then obviously fewer pulses will suffice.)
Having just done a quick search, beware: “digital” does not imply a pulse output.
For installation, all you need (if it’s a short run) is a pair of telephone-type wires from the meter to your emonPi. If it’s a long run, then a microphone-type screened cable might be necessary, to reduce the likelihood of counting interference as oil!