Why doesn’t oil create as much condensation as gas when burned?
Actually it does. Oil burns at higher temperatures.
It is the dew point of the combustion gas that creates the visible collection of condensation.
Gases exiting the chimney above the dew point of the flue gas produce condensation after termination from the flue vent. This is the steam that is present above the chimney at the exterior of the home.
In firing Heavy Industrial Furnaces/Boilers, the visible tail of SOX, NOX, etc… emmissions could be controlled by the monitoring the stack temperatures as well as other variables. Emmissions may not change but the visible appearance can be altered and adjusted.