Valvoline 4 stroke motorcycle oil can be found at Auto Zone amoung others. Fairly economical JASO dino.
As far as M1 15-50, It works well as noted but it will shear to a 40 pretty fast, even in non shared sumps. I have used it in my Harley evo but still keep the OCIs short due to shearing and low miles / long durations in that bike as it is not a primary rider. The wear numbers from UOAs still look good even with the shearing. Not sure why it shears when the M1 20-50 V-twin does not other than to conclude that it my be a Group III with VI improvers. Something in the base formula makes the V-twin and 4T oils much more shear stable even thought the additive packages are quite similar. PAO instead of GIII???? Twice the cost though over 15-50 auto oil though. 15-50 usually shows about 90 ppm moly but most say clutch operation is fine at that level.
As far as M1 15-50, It works well as noted but it will shear to a 40 pretty fast, even in non shared sumps. I have used it in my Harley evo but still keep the OCIs short due to shearing and low miles / long durations in that bike as it is not a primary rider. The wear numbers from UOAs still look good even with the shearing. Not sure why it shears when the M1 20-50 V-twin does not other than to conclude that it my be a Group III with VI improvers. Something in the base formula makes the V-twin and 4T oils much more shear stable even thought the additive packages are quite similar. PAO instead of GIII???? Twice the cost though over 15-50 auto oil though. 15-50 usually shows about 90 ppm moly but most say clutch operation is fine at that level.