The use of the electric motor (EM) inside the transmission in electric vehicles (EVs) requires compatibility between the automatic transmission fluids (ATFs) and the materials of the EM and the transmission. The goal of this work is to study the compatibility of four conventional ATFs with three structural polymers (PEEK, PTFE, and PA66) and the feasibility of using them in EVs with the EM located inside the transmission. Changes in volume, hardness, tensile strength, and elongation at break were determined in the polymers after ageing in the ATFs. The polymers were aged in each ATF at 100 °C for periods of time of 168, 336, 504, and 672 h. Complementary tests such as XRD, FT-IR, and DSC were performed to explain the changes found in the measured mechanical properties. PEEK and PTFE showed very low variations in their mechanical properties, while PA66 showed significant changes in elongation at break. The XRD, FT-IR, and DSC tests revealed a change in the crystalline structure of PEEK and PA66. The FT-IR results showed that the polymers were affected more by the time of ageing than by the type of ATF. All of the ATFs showed high compatibility with PEEK and PTFE, and medium compatibility with PA66.