We collected data focusing on in-use emissions and efficiency of engines servicing the unconventional well development industry to elucidate real world impacts from current and newly applied engine technologies. The engines examined during the campaigns were diesel only (DO) and dual fuel (DF) diesel/natural gas, compression-ignition (CI) engines and dedicated natural gas, spark-ignition (SI) engines. These included two CI drilling engines outfitted with two different DF kits, two SI drilling engines, and two CI well stimulation engines. Our data were gathered under the load and speed requirements in the field, and the engines were not under our direct control. Greenhouse gas (GHG) emissions were measured from all engines and fueling types and included both exhaust and crankcase emissions. Fuel consumption and engine data were collected to determine fuel efficiency. During steady-state operation, fuel efficiency was 38%, 26%, and 20% for DO, DF, and SI engines, respectively. The loss of efficiency during DF operation was due in part to uncombusted methane (CH4) slip in the exhaust, which accounted for 18% of the fuel supplied. GHG. emissions (carbon dioxide and CH4) from CI engines were 2.25 times higher during DF compared to DO operation. During DF operation, substitution ratio varied depending on engine load and DF kit, ranging from 9% to 74%. GHG emissions from the SI engines were 1.33 times higher than DO due to lower efficiencies of throttled and rich operation as compared to unthrottled and lean operation for CI engines.