I know the secret too, I was working there for $3.35/hr when we switched over to the new oil they use.
Double cheeseburgers were not on the daily menu, but were sold a couple times a year, and the double quarter pounder was not sold at all, even though we made them for ourselves. 
The secret for the fires was Beef Tallow
Fifty pound cubes of vegetable based oil. Did many oil changes while there.
The secret was in the prep.
Here it is.
Wash, peal, wash, slice, wash, wash again.
Hang and let drain.
Put basket in oil for 30 seconds, remove and let drain and cool to room temp.
Only now, do you take the basket and put back in the hot oil. 350 degrees on both steps.
That's all there is to it.
The thirty second blanching then cooling, was the key.
Yes, BEG, you're right.