Petroleum-derived spray oils (PDSOs) have been used for pest management in horticulture and agronomy for over a century. Civitas™ is a new PDSO designed for use in the turfgrass industry. It is commonly mixed with low rates of pesticides to reduce the environmental impact and improve plant stress tolerance. Civitas can cause phytotoxicity, which has limited its acceptance by the turfgrass industry. Civitas is mixed with a green pigment called Harmonizer™ to sustain acceptable turfgrass color. A field study and a growth chamber study were designed to quantify phytotoxicity, understand the role of Harmonizer, and isolate the cause of Civitas-induced phytotoxicity. Civitas, Harmonizer, their combination (Two-Pack), and a water-only control were applied to a research putting surface in Ithaca, NY, during 2012 and 2013. Civitas and Harmonizer were applied every 2 weeks at the rates of 5.0 and 0.3 mL·m−2, respectively. Visual turfgrass quality rating and canopy temperature were quantified several times weekly. Civitas caused chlorosis and decline in visual quality during both years. Harmonizer masked chlorosis but did not prevent a drop in stand density during the second field season. Treatments were replicated on annual bluegrass (Poa annua L.) in a growth chamber experiment. Civitas did not increase electrolyte leakage or alter the composition of cuticle; however, there were signs of oil persistence on the leaves and stomata and evidence of reduced gas exchange. Chlorosis resulting from oil persistence and reduced gas exchange is consistent with chronic PDSO phytotoxicity. This research demonstrated the potential for phytotoxicity with high rates of Civitas. Lower application rates likely reduce the potential for phytotoxicity but may also minimize the pest control benefits associated with the product.