Amsoil, Red Line and Mobil1, have been rated as the top syn oils by several independent studies, the one I have is a few years old, but probably hasn't changed much. Also the ratings move around depending on the viscosity being tested.
A few weeks ago I went to my local shop to get some MTL, they didn't have it, but had Red Line Primary Case Oil, they said that Red Line now recommends for the primary. So I bought some, I think the primary gets to much attention when it comes to the oil, there very little lubrication going on here, mostly heat dissipation. Below is the description from the Red Line web page.
V-Twin Primary Case Oil
Red Line’s new V-twin Primary Case Oil is designed specifically for ’84-to-present V-Twin American motorcycle engines, including aftermarket engines of similar designs. Similar to our MTL, this product is for use in the primary chaincase with wet-diaphragm spring clutches.
Designed to operate over a wide temperature range, this Primary Case Oil improves clutch operation while reducing wear and temperature. Where most synthetics are too slippery for clutch operation, this friction-balanced product is balanced to provide less slip, smoother operation, and helps to apply more power in modified engines.