I am currently running Mobils synthetic blend for high milge engines, i wanna go full synthetic but the guys at outo zone say that my car has to many miles on it, i dont have any oil leaks, so i dont see what the problem is. i have 140K on it, and the switch to part synthecitc didnt cuase any leaks, so can i?

Also i guess once you go synthetic you cant go back? is that true, and is there also any prep work that needs to be done before switching to syntetic?