It's the oil smell that removes the APU and Packs from contention in this bout. The oil (and fuel) smell after startup is the first indication of a worn Number 3 Engine Bearing Rear Seal.
Other possibliities include...
Oil contamination at HPC stages 3 and 4, or the 7th stage Bleed Valve.
When any of these happen, the oil smell will be noticed during startup, and to a lessor degree when the engine is operated at Idle (for example, during decent).
Other than the smell, this is not a great concern INITIALLY- except to the airline accountant who can expect to be paying for the removal of the engine in order to repair the problem.