I'd look at the higher temp cst ratings on the oil rather than just the oil weight rating.
I personally use a 0w40 or 5w40 on my jdm pulsar gti-r.
for example
oil, weight, 40 deg C Cst, 100 deg C cst
Mobil1, 0w40, 78.3, 14 <-- previously used
Shell Rotella T Synthetic, 5w40, 95, 15.5 <-- my current oil, this oil doesn't seem to break down as quick as the mobil
Esso XD-3, 0w40, 87, 15.2
I have compiled a long list of numbers but I don't know how its going to turn out when I cut and paste it from excel onto here.
Shell Rotella T Synthetic 5w40 95 15.5
Shell Rotella T Synthetic 15w40 120 15.5
Mobil1 0w40 78.3 14
Mobil1 10w30 62 10
Mobil1 5w30 64.8 11.3
Mobil1 5w50 105 17.3
Mobil1 Advanced Fuel Economy 0w30 63.1 11
Amsoil 0w30 56.6 10.3
Amsoil 0w20 49.1 9
Amsoil 5w30 60.7 10.8
Amsoil euro 5w40 79.7 13.7
Motul 300V Power 5w40 80.8 13.8
Motul 300V Chrono 10w40 89.5 14
Motul 300V Power Racing 5w30 65 11
Esso XD-3 0w40 87 15.2
Esso XD-3 0w30 69 12.2
Elf Excellium 10w50 119.7 17.9
Elf Excellium 5w50 115 18.2
Elf Excellium GM 5w30 70 11.8
Elf Excellium XLL 5w40 83.93 13.82
Elf Excellium Full Tech 0w30 68.45 12.18
Elf Sport TXI 5w40 85.11 14.05
Elf Excellium NF 5w40 85.11 14.05
Castrol doesnt list the 40 degreee C Cst value, only the 100
Castrol Syntec 0w20 8.8
Castrol Syntec 0w30 12.1
Castrol Syntec 5w20 9
Castrol Syntec 5w30 9.7
Castrol Syntec 5w40 14
Castrol Syntec 5w50 17.4
Castrol Syntec 10w30 10.1
Castrol Syntec 10w40 13.2
Castrol Syntec 20w50 19.7