The white etching crack (WEC) behavior of five model lubricants and one fully formulated gear oil has been investigated using FE8 test rig. Varying concentrations of branched ZDDP (zinc dialkydithiophosphate) and OBCaSu (overbased calcium sulfonate) have been studied. The results reveal that high concentration of branched ZDDP and OBCaSu is critical to WEC failure, while the low concentration not. This is attributed to tribofilm structure, composition and mechanical properties. Tribofilms formed from less critical lubricants show up to 110% higher hardness and 80% higher elastic modulus than the WEC-critical lubricants. Rapid tribofilm growth and higher amounts of certain tribofilm species like FeO, CaO and ZnO appear to have a weak tribofilm structure that potentially could promote hydrogen permeation. FeO in tribofilm is considered to act as catalytic sites for hydrogen dissociation, leading potentially to hydrogen diffusion. Changing from a branched to linear ZDDP postpones WEC significantly but did not prevent it. Replacing a bad reference WEC-critical oil with a good reference oil could postpone WEC failure but cannot be prevented. The fully formulated gear oil that formed thin tribofilm and less-critical tribofilm species did not show WEC. Special MPR tests made using specimens with non-metallic inclusions showed an earlier WEC failure than those with standard specimens without such non-metallic inclusions, suggesting that non-metallic inclusions could promote WEC even in the absence of WEC-critical additives and water.