My oil cooler looks like this..(photo courtesy of slickroger)

So I need to replace the oil cooler gasket ...
It doesn't look like too bad of a job. Just wanted to know if when you change the gasket does all the oil drain out?
The reason why I am asking is the oil that is currently in there is brand new and would hate to waste it.
I figured if it does drain all the oil I will just lump this gasket job in with my next oil change... or if it doesn't drain all of the oil then I would just do it immediately. I had the under cover off anyway dealing with alternator/amp issues.
