True, and not necessarily true. I've seen vehicles leave the dealership and go for a drive after an oil change and come back with the oil light on....never filled with oil.When your mechanic said this " Mechanic said oil all gone, dipstick completely dry. "
He is full of poop. If all the oil was gone it wouldn't have ran 5 minutes. Just because it isn't touching the dipstick doesn't mean it is empty.
As many have said leaking or burning, its gotta be going somewhere. It can get burned a little at a time over the course of 6 months and be low and off the dipstick without any obvious signs.
If it were leaking you would have oil on the engine somewhere telling as much.
I've personally run one for 15-20 minutes while cleaning the injectors....I had done a complete 30k mile service on it. Oil change, coolant, tranny fluid, brakes, plugs, fuel filter, etc.
edit: This was a 92 Explorer, 4.0
It was a Friday and it was my last job for the day. Tool truck was there, so I hooked up the injector cleaner and let it run, went on the tool truck for about half an hour. The cleaner had enough fuel to run for 15-20 minutes.
Came back, it had run out of fuel and shut off. Unhooked the cleaner, buttoned everything up, went to road test it and go home. As I backed out of my stall, I noticed a red light on the dash. I looked up over the hood and on my bench was 5 qts of unopened oil. Oops.
I shut it off. Engine hadn't made a sound yet. I put the oil in it, still no noise. Okay, so it's still under warranty, I'll go drive it and see how it does. Ran the hell out of it for about 10 miles. No noise. I let it go.
2 years later, I did the 60k mile service on it. Still running fine.
They don't always fail like you think they might without oil.
That said, for OP....if you're not leaking, it's consuming the oil. That can happen. You are responsible for checking it. And not showing on the dip stick doesn't mean there's no oil in it, it usually only means you're probably 2 qts low. Which isn't good, but it's not going to kill your motor, either.