Been having a discussion in the Motorcycle section about using M1 15w50 instead M1 V-Twin 20w50 (especially in Harleys). Seems as though my only concern is the potential for quick shearing out of grade with the 15w50 in that it is a low 50 weight (cSt @ 100C 17.4 per the MSDS) while V-Twin is a high 50 weight (cSt @ 100C 20.8 per the MSDS). [Both of Harley's 20w50 oils are high 50's.] Patman stated in another thread that the 15w50 has a HT/HS of 5.11, which I take it is a good indication of it's shear stability. However, a UOA of M1 Tri-Syn 15w50 in an 89 BMW 2.5L w/5,024 miles on oil posted by Jason Troxell indicated a SUS @ 210F 73.2 (low 40 weight). If the 5.11 HT/HS is for the Supersyn variety, does this suggest that it sould stay in grade, even in an air cooled motorcycle in the dead of summer? The 15w50 has a cSt @ 40C of 125.0 (per the MSDS) so does this suggest that maybe if it does shear it will not go below a 40 weight?