ABSTRACT The influence of a helicopter gearbox lubricant on Mode I or Mode II fatigue crack growth in 16NCD13 steel was characterized through tests performed on single‐edge notched tensile samples loaded in tension–compression and on cruciform samples submitted to reversed shear plus static biaxial compression, respectively. In Mode I, the lubricant reduced the growth rate at low Δ K I and increased the threshold , while in Mode II, it accelerated crack growth at low Δ K II effective , which was not only due to a reduction in crack face friction. The upward convective flow of lubricant carrying debris exuding from the crack, a modification in oil aspect and properties, and chemical analyses near the crack front suggest that a temperature‐induced degradation of the lubricant leads to a corrosive attack of the metal, which accelerates crack growth. A tribologically transformed structure is observed along the lips of cracks grown in Mode II with normal compression in oil.