Civil aviation gas turbine engines emit ultrafine particles (UFPs, diameter < 100 nm). UFPs degrade air quality because they efficiently transport their chemical content, including engine oil, into the body. Yet, little is known about how and when oil containing UFPs are formed. Results here describe the thrust and flight phase-dependent oil emission and reveal the particle size-dependent transfer of vaporized oil to UFPs with molecular level resolution. All six engines studied emitted oil containing UFPs. Lower volatility oil molecules are enriched on particles <30 nm. Further, the particulate oil mass size distribution aligns with the emitted surface area distribution, suggesting oil vapor condensation onto primary particles and the potential for oil nucleation. However, the oil gas-to-particle transfer in hot exhaust is likely incomplete at least 50 m downwind thus limiting current emission studies. The measured engine oil consumption provides an upper-limit oil emission index at idle of 240 mg oil per kg fuel. The emission index at cruise is 110 mg kg^–1, which is a factor of 10 greater than black carbon. For any flight >2 h, 95% of oil emission is expected to occur at cruise altitude, highlighting the unknown effects of oil emission in the upper atmosphere.