There are eight logging facilities commonly used for syslog. They are local0 through local7. Each one of these 8 facilities have a numeric value equivalent to local# on syslog server.
( For more information on the facility levels on the PIXes, please refer to the following document:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a0080094030.shtml#logfacility)
Since you want to log your PIX INFORMATIONAL syslog messages to /var/log/pix.log, you need to change your logging facility to 21 (on your PIX), which is equivalent to local5 facility
Try "logging facility 21" and see if it works for you.
Thanks,
Binh