Concerning conventional oil:
Multi-viscosity oil contains a polymeric Viscosity Index Improver. This special additive is a coiled molecule that uncoils when heated. It literally becomes larger and acts as a thickening agent, somewhat like adding flour to gravy. Unlike flour, however, as the molecule cools, it returns to its smaller shape. Unfortunately, the VII is susceptible to shear forces. When subjected to extreme pressure or shear, the molecule can be sliced apart. It then become two smaller molecules. As the smaller molecule is subjected to more and more shear, it begins to breakdown and oxidize, at which point it start settling out of the oil. Hopefully it’s captured by the filter. If it isn’t, it settles into low spots in the engine adding to the sludge problem.
I read somewhere that when using a wide viscosity conventional oil that the additives in it contribute to sludge issues.
I now use 10W-30 Castrol conventional in my older vehicles. Synthetic in our late models.
