If the heat source is 23W I'd expect the balance to be around -23W as more heat leaves than enters through the boundaries. That more-or-less ties up with the graphs.
If you check the (total imbalance * cp) what sort of values do you come out with? Temperature may not change much if most of the heat finishes up in the oil as it'll have a relatively high heat capacity compared to air.
The domain oil fraction isn't looking to be entirely at equilibrium, but I appreciate this is shorter plot than the whole simulation.
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