mad_jock - I have to disagree and am in Offchocks' camp on this one. Any F/O flying in a commercial aircraft, with 165 hours with a fATPL or 2,500 hours with a full ATPL, cannot log any of their flying as PIC because they are not the PIC, so what's your alternative?
In my commercial logbook I have "Dual", "PIC", "P1 U/S" and "Co-Pilot" columns, so back in my F/O days, when I happened to be PF for a particular sector, I did as Offchocks did and logged the time as P1 U/S and asked the PIC to countersign the entry as per the logbook's instructions. When operating as PNF I logged the time as Co-Pilot, or P2 as in the case of some logbooks.
I couldn't log PF time as PIC because I wasn't the Pilot in Command and it would be misleading, nay dishonest, to log it as such, and as stated above Co-Pilot status applies to PNF sectors. Whilst PF I was P1 (not to be confused with PIC) under the supervision of the Pilot in Command, so logically the time was logged as P1 U/S and I'm happy with that.