Here is what has worked for me.
Spray some carb cleaner in the hex on the plug and muck around with a small screwdriver to make sure the fitting is absolutely clean. You need to be able to get as much of the wrench as possible in.
Then spray some PB Blaster around the threads and let it sit an hour, giving it a good tap every once in a while.
Then use a good quaility allen wrench with a perfect end - buy a new one if you have to. Make sure it is inserted perfectly straight. I have found the wrenches get a better "bite" than the sockets. Slip a piece of pipe over it for more leverage. You will feel it flex and then a "pop" as it lets go.
BTW - assume you are removing the tensioner arm to change the lining? On the 116/117, you can change the just the lining and I think you can do it without removing the arm.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe