I am not sure why this question always seems to come up.
It is a simple answer, and spelt out in the FAA and ICAO regs. if you are not designated as a PIC, in your company, and on the trip authority then you CANNOT log PIC time. If you are designated by your company as a F/O. You CANNOT log PIC time. If you are not the PIC on the flight but are the PF, You CANNOT log PIC time. The fact that you are the pilot flying in a multi crew aeroplane does not automatically make you the PIC. The PIC is a designation, he is the Pilot in Command, not the Pilot flying.
And before you all start making excuses about CFI's and receiving duel, under the hood etc. We are talking about a MULTI CREW ON DEMAND/SCHEDULED COMMERCIAL OPERATION. There can be only 1 PIC.
PICUS is a different story