The tool is called a "drain plug socket", and yes it is 5/16. They can be bought individually, or in a set of various sizes. Get one(s) rated for impact use...they dont break as easy. Grinding down a 3/8" adapter or extension didn't work for me...broke almost instantly!
http://www.ebay.com/itm/Grey-Pneuma...t-/190740964619?ssPageName=ADME:X:RTQ:US:1123
You'll also find the plugs on the rear of the block are the same thing.
