Bad compression rings can allow gasoline to get into the oil.
Crude oil is typically removed from very high pressure reservoirs and is made up from a range of different hydrocarbon chains. Gas as you refer would mainly refer to methane which is CH4 and oil as...
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Carbon Dioxide (CO2) Sulfur dioxide (SO2) Nitrogen Oxide (NOX) Carbon Monoxide (CO) Non-methane Volatile Organic Compounds (NMVOC) Methane (CH4)
Oil can be used in various motors or engines to power them along and gas can be used on a stove to start a fire.