Many drivers assume viscosity alone controls shift feel in automatics. In reality, friction modifiers set clutch behavior at every slip event. Mixing small amounts of incompatible fluids can change the friction curve. That may cause delayed engagement, shudder, or harsh shifts after service. Viscosity still matters, but only within the control system’s design range. Cold starts raise static friction, which confuses self-learning shift logic (adaptive) in winter. Gentle early driving warms parts faster than idling, improving engagement stability. Used oil analysis (lab report) can confirm oxidation, shear, and additive loss. Choose a fluid with stable friction and correct compatibility, such as Multi Vehicle ATF (TYPE T-IV).
