MCN did an excellent test and write-up on oils back in '03 but I can't find the issue it was in right now. They tested about 30 oils from several different makers - standard petro and synthetic, motorcycle specific and standard automotive.
The testing was pretty comprehensive and what I took from it was; there are two major things an oil must do in addition to lubrication (or in order to maintain good properties) disperse heat and not loose viscosity, and neutralize acid build-up from combustion. Some oils are better at one, some another. The racing oils (expensive oils from Silkolene and Motul) tended to do a very good job of handling heat but were no better then middle-of-the-pack for acid neutralization. This sort of makes sense since race bikes get frequent oil changes - often after every race and handling heat is much more important for this type of application. Dealing with acid build up is more important for regular street bikes since we (most of us)only change our oil once or twice a year and we'd rather not have a lot of acid floating around in our crank cases in between.
The Mobil MX-4T product scored in the top 10 for both and I've been running this in my Trophy now for years with good results. Amsoil scored in the top 5 in each catagory making it the highest scoring oil overall when both issues were considered.
Their conclusion supports the info I've read in several other articles:
1. Change your oil regularly - despite what manufacturers and oil companies say 6-7k between oil changes is a long time for small high reving motorcycle engines.
2. Motorcycle specific oils perform measurably better in motorcycles than almost all car oils.
3. Fully synthetic oils performed measurably better than standard petroleum oils and most blends (semi-synth.)
4. If you are going to go 5-6k between oil changes a fully synthetic motorcycle oil is a must.
I run Amsoil in my BA figuring at about $7 a quart (last time I bought it) it is about as good as I'm going to do for regular street riding.
Keith
