Its 14mm and use a wrench with a long handle, it will snap free so watch your hands. Same goes for the inner oil mesh filter. That's a long mesh tubular type affair, again using same 14mm head.
After inspection and cleaning put the lot back together using a short arm wrench. Have heard of people applying too much torque and stripping alloy threads. Veeery expensive.....
Good luck.
ps this is on a 2001 996
[Edited on 13-2-2004 by Bianchi]