You should put oil on your meat to prevent it drying out. Does it make a difference if you coat then cook straight away or leave coated for a while, if so how long should you keep it for?
You actually want the outside of the meat to dry out: the drier it is the faster you'll get a brown crust on it (Maillard reaction) and more juice will stay on the inside. Oiling the outside won't prevent it from drying inside if you overcook it.
You don't need to coat the outside in oil before grilling, you just need to keep an eye on it when grilling so it doesn't stay there for a long time just dehydrating.