My 560SL has the 14mm hex drain plug. It's not an aftermarket plug. I've only had the 300SEL for less than a month, just got the oil filters in, so I'll be changing it this weekend. Haven't checked the drain plug on that one yet.
Been to two different AutoZones (which my non-Mercedes mechanic hates), as well as many other auto parts stores, Sears, and Home Depot. Can't find them anywhere. I've seen a Hazet online for ~$16. 14mm only.
Yes, I still need to solve the rear axle lubricant problem. I think the interval on that is 100,000 miles, so I might just take that to my mechanic instead of buying the tools. Or maybe not. Haven't decided yet.
In the meantime, I've already used my Topsider a few times and will again this weekend. Since the filter on my 560SL is underneath, I still have to get under the car (and do a visual inspection).
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino