Extreme Pressure (EP) is the most common anti-wear additive for high load gears. It is Sulphur-Phosphorus based and used when other additives such as ZDDP are not sufficient to give high load wear protection.
If you have ever changed a gear oil, the distinctive smell is from EP.
In normal conditions EP is inactive, but at high temperatures at loaded surfaces EP reacts to form a metal sulphide toughened coating to protect against wear.
However with copper based components care must be taken as EP is so powerful it can cause corrosion. Hence always be guided by your oil supplier & OEM for correct lubricant choices.
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