Cover the allen key/socket with a paper towel and then stick it into the allen head. The papertowel provides just enough fill to take out any slack that could lead to stripping.

I did this with the filler to my differential that hadn't been moved in 20 years. With just the allen socket, it would have stripped, using the paper towel, I used a 2 foot breaker bar and pushed it with both feet and it came loose without stripping.

Good luck,

JCD