Right, but there may be more to the desired interaction set than the ester component of the zinc compound. If the zinc compound interacts with surface metal atoms, what is the nature of the interaction? Is the sulfur, zinc, phosphorous or hydrocarbon actually adsorbing to the surface? Sulfur can interact with metal surfaces to create a new phase or material with substantially different properties (one of the reasons, for example, that sulfur made a good lubricant in diesel was that it interacted with components in the alloy used in typical injection pumps. This alloy had a completely different character (hardness, melting point, etc.) than the bulk metal, and caused a wear reduction.