The reputation is that 10W-40 oils break down quickly. Some think the 30 point viscosity spread requires greater VIs, which shear quickly, losing viscosity. 15W-40 oils get high praise for resistance to shearing and breakdown. They have a 25 point viscosity spread, 5 less than 10W-40. Several questions: Is there really that much difference between these oils? Does the additional VIs required to get 5 more points make the 10W-40 dramatically more susceptable to shearing? Is the additive package that much more robust in 15W-40? Is the main difference between 10W-40 and 15W-50 greater amounts of ZDDP in 15W-50? Is the bad reputation of 10W-40 based on older oils, and are modern 10W-40's very good like most oils. Why does 10W-40 have such a bad reputation?