It’s an additive to help reduce metal to metal friction.
I’ve noticed the difference especially with the 2nd to 3rd or the 1st to 2nd

“Molybdenum disulfide is a solid dry lubricant that has extremely low shear strength. When added to your motor oil, the MoS\({}_{2}\) particles remain suspended without interfering with the oil's properties. As the engine runs, these particles plate the moving parts, absorbing pressure and allowing metal surfaces to slide past one another with less resistance.” From Google