Make sure the tool you got is made of metal, actually fits snug, and is the cap kind. I had this problem on my dad's w140 s320. The plastic tool snapped but the metal one worked. Here's my thread from last year:
http://www.peachparts.com/shopforum/showthread.php?t=249993

(my engine was warm, too, which I believe helped)

just FYI, I used a lot of brute force even with the made to order exetender pipe. But do exercise caution and need to use some finesse. Just be careful