I work with heavy machinery next salt water so I deal with stuck nuts and bolts all the time. Get a steel rod about 3/4 in. dia. or slightly bigger. Cut it to about 6to8 in. long. Make sure the end cut is square. Get a 2 lb. shop hammer. Place the steel bar square on top of the allen bolt and hit it as hard as you can 2 times. Insert the allen wrench with a cheater pipe on it or use an allen socket with a break over bar (14 to 18 in.) and loosen it. Don't try to hit the top of a bent allen wrench. It won't work and it will hurt your hand.
Rapping the bolt with a hard, sharp blow sends a kind of shock wave down the bolt that fractures any kind of corrosion (not likely with a bolt that is bathed in oil) or lock tight. Don't use a socket extension as a drift. The male square end is usually not cut square, it has a slightly rounded end, and this will tend to dimple in the hole. This will make it hard to get the allen wrench in. If you do this it is nearly impossible to file/grind open the hole. Don't use a brass drift either. What you want is the steel on steel shock effect. Brass is soft and will tend to absorb the blow. Don't bother with wd40 or the likes. If, over the years of running, hot engine oil has not penetrated into the threads a shot of wd40 doesn't stand a chance.
Be sure that you have the exact size allen wrench and that it is not rounded on the corners. Once the wrench spins in the bolt you are basically a done duck. Then you either take it to a old time mechanic who will see your situation and subsiquently screw your eyes out or, if you know how (in which case we would not be having this discussion), get out your welding machine and weld a grade 10 cap screw to the top of the allen bolt and use an impact to back the bolt out.
Remember too, you can put a lot more torque on that bolt that the tighening force recommended by MB. Allen bolts are usually about grade 10 and are really tough.