Don multigrades gradually get thinner and cause oil consumption?
Not these days. The “thickeners” (Viscosity Index Improvers) are now very stable. Also, these “thickeners” or polymers also can give additional elasticity to the oil resulting in thicker oil films than with a monograde. My oil pressure is lower with your new oil! Assuming the oil pressure gauge remains accurate, it is measuring pressure in the oil gallery after the positive displacement oil pump. This pump moves a constant volume of oil at any given engine speed. Assuming that there is no major leakage from a badly worn main bearing or even an oil pump, any variation in pressure is the result of a change in the resistance to the flow of oil. This could, among other things, be a result of inherently lower friction from the oil because of some friction modifying property, or a greater resistance to oil thickening either from soot or aging (oxidation). Whatever the reason, the same amount of oil is still circulating and the gallery pressure does not affect the pressure in the main or big
