There are several answers for this question, although the main effect that we can find with friction reducing on an engine is reducing the heat transfer between the moving parts.
Reducing the friction between two moving parts means reducing the heat transfer. One of the more important effects of the engine oil is to reduce the friction between the moving parts, which means reducing the heat generation or heat transfer, and it's especially true when we are talking about synthetic oil (synthetic engine oils can resist heat transfer better than traditional engine oils).