Shearability of M1 15w50

LA (Lower Alabama)
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?


Staff member
Oakville, Ontario
Having a high HTHS does not mean the oil is less likely to thin out over your interval though. It simply means it won't thin out as much as other oils as it gets hotter and is put under more stress. In other words, if you take two oils of similar viscosity at 100c, but one has an HTHS of 3.2 and the other is 3.8, the one at 3.8 won't thin out as much under extreme heat and pressure as the lower one would. But it's very possible for the oil with the HTHS of 3.2 to remain closer to it's virgin viscosity than the other one. Keep in mind, if an oil thins out over it's interval, it's HTHS then also drops. So you take that Mobil 1 15w50, which has it's HTHS of 5.11 when new, but as the oil thins out into a low 40wt, it's HTHS probably then becomes closer to 4.0 or lower.