Then please explain what the two numbers either side of the w mean. Is it not true that the higher the number the more resistance to flow (internal friction or viscosity) hence using 20w50 in older low revving engines?
My understanding is that simple fluids decrease viscosity (get thinner) with temperature fairly linearly. Hence traditional oils have a single viscosity number. Friction modifiers were introduced as a clever way of reversing this trend for a range of the temperature. Ah the wonders of modern science.
Happy to be re-educated.
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