Viscosity index improvers are part of the additive package blended into base oil to produce engine oil.
Your old mechanic teacher may have had an element of truth in his statement many years ago, but that hasn't been true for many years. Except for engines operated in constant service in (industrial engines, for example), multi-vis oils provide much better flow and consequent better lubrication until fully warmed.
Truck and construction equipment engines use 15W-40 (except old Detroit 2-strokes). Railroad, tugboat, and generator engines use straight 40 wt. Different oils for different usages. Also, except in specialized oils for certain applications, 15W-40 has the best, up-to-date formulation.
Ken