not sure if this is old news, but both shell and exxon/mobil have webpages which suggest specific oils for specific cars.
The Shell global homepage - Shell Global
What's the right oil for my car?
FWIW, for my car (1981 911sc), shell recommends 5W-40 full synthetic, and exxon recommends 0W-40 full synthetic. (the shell website doesnt include the 1981sc, so i used the 1984 carrera instead.)
edit...castrol UK has a similar webpage and suggests a full synthetic, either 0W-40 or 10W-40
http://www.castrol.com/castrol/sectionbodycopy.do?categoryId=9024084&contentId=7073092
castrol US has a similar page which includes different suggestions for different climates, and suggests 20W-50 above 14*
http://www.castrol.com/castrol/LubesSelector.do?categoryId=3240