The answer to your question may vary depending on the oil pan and the oil pan drain plug that is on your car. Earliest W113 cars used a 17mm hex bolt type head drain plug. Next came the 14mm allen wrench type drain plug. Lastly came the 13mm hex bolt head type drain plug used only on any new replacement oil pans.
I suspect yours is the 14mm allen style drain plug. There is no special tool provided in the tool kit for some reason. A generation earlier, in the 190SLs and 300SLs these special 14mm drain plug wrenches were included in the factory tool kits which came with the car. It is a handy tool to have since it fits the drain plug on the rear diff and the standard transmissions also. A standard 14mm allen wrench or even a 9/16" allen wrench will work fine. You can also purchase a socket with a 14mm allen stud to use on your ratchet or breaker bar. I suggest that you get the 1/2" drive 14mm for best results. Any good tool supplier will have them these days.........Snap-on, Craftsman (Sears), Cobalt (Lowes), Harbor Freight etc.
Mercedes sold an optional Lubrication Service Tool Kit you could order back then which had the special tool in it along with lots of other nice service tools, like the Merceces-Benz chrome tire gauge and the tool for lubricating the door hinges. These are rare and hard to come by these days. However you can order the special wrench by itself from the dealer (MB part number 000 589 07 00). It is made by Hazet (#2760). You can often turn one up on Ebay. This special wrench has the 14mm allen on one end and a six point 17mm box end on the other end. This tool will work very nicely on the early 17mm bolt type drain plugs and the 14mm allen type drain plugs.
While your changing your oil, make sure that the upper oil housing rubber seal is in place. These are critical and are often missing. Go to the Tech Manual or search up "oil change" information to understand this subject. It is important!
