Zinc dialkyldithiophosphates (ZDDPs) are widely used additives in automotive lubricants which form crucial antiwear tribofilms at sliding interfaces. The mechanisms governing the tribofilm growth are not well-understood, limiting the development of replacements with better performance and catalytic converter compatibility. Using atomic force microscopy in ZDDP-containing lubricant base stock at elevated temperatures, we monitor the growth and properties of the tribofilms in situ in well-defined single-asperity sliding nanocontacts. Surface-based nucleation, growth, and thickness saturation of patchy tribofilms are observed versus sliding time. The growth rate increases exponentially with either applied compressive stress or temperature, consistent with a thermally-activated, stress-assisted reaction rate model. The films grow regardless of the presence of iron on either the tip or substrate, highlighting the critical role of stress and thermal activation.