Selecting the correct fluid for a is critical, as these units—commonly found in Ford heavy-duty trucks (S5-42/S5-47) and various BMW models (E36/E46)—are known for being robust but sensitive to fluid viscosity. While original factory requirements often specify synthetic ATF, high-mileage users frequently turn to specialty manual transmission fluids (MTF) to address synchronizer wear and "notchiness". Top Fluid Recommendations & Reviews