Originally Posted by
WEACLRS
...So if the aircraft requires two pilots by it's FAA type certificate or if the flight requires two pilots under the FAA regulations it is flying under, and you are rated (category, class, and type), and you find yourself in the right seat, and its a part 91 flight, you can log SIC, flight time, day/night, cross-country, but not instrument...
I'm incorrect here. As an SIC you may log the condition of the flight, including instrument conditions. You may not log the approach for currency requirements unless you actually manipulated the controls (it was your leg).
See the following FAA interpretation (it's a bit dated, but still applies):
http://www.faa.gov/about/office_org/.../Carpenter.rtf