This is always a contentious topic on any motorcycle board. If you do a search on here you'll find a number of threads full of arguments why each type is "the best." However, you'll not find any conclusive proof. In the end, if you use a quality oil in the weight specified for your bike (probably 10W40 or 10W50) and change it at the specified intervals, you'll never have an oil-related problem. I use Shell Rotella 5W40 synthetic diesel oil, as do many others. Some even use the 15W40 Rotella diesel oil with great success. Others swear by Mobil 1, Amsoil, or Royal Purple, but a motorcycle-specific oil is not necessary - any oil of the proper weight will be fine. Just change it regularly. Remember that none of the m/c manufacturers refine oil, so their oil is a repackaged version of something else and therefore not a good deal. The same applies to filters.
for some interesting reading on oil and filters. Be sure to check out the other sections on the LH side of the page for info on filters too.
"We didn't trade shoves; I did all the shoving." -Bobby Knight, 1981