term you reduce the superior antiwear properties of synthetic oil, which therefore, increases engine wear. Synthetic oil is engineered to be stable over a wide temperature range which translates to stable viscosity. You get the same degree of protection when the oil is hot just as when it is cold. For dino oil (or regular oil,) modifiers are added in an attempt to prolong stability over a wide temp range so that when it is cold the oil doesn't get too thick, and when it's hot it doesn't get too thin. Such stability, however, is short-lived.
Dino oil hurts the engine during cold start because the oil is thick and there is hardly any lubrication available which causes engine wear. When the oil is hot it becomes very thin and you get nearly metal-to-metal contact which also causes engine wear.
With synthetic oil, you enjoy it's benefits because the oil is stable over a wide temp range for a long time. You get the same thickness, and therefore same level of protection whether the engine is hot or cold... the engine runs cooler and engine parts don't wear out as quickly.
Having said all this, mixing synthetic with regular oil reduces the effectiveness of the former oil to combat engine wear.