Richard Stribeck published his measurements of bearing friction in 1902. The curve that carries his name is still the first diagram a lubrication engineer learns, because it shows in one line how friction depends on viscosity, speed and load.
Three regimes
On the left of the curve, speed is low or load is high. The oil film is thinner than the roughness of the surfaces, metal touches metal and friction is high. This is boundary lubrication. In the middle, a film begins to carry part of the load and friction drops quickly: mixed lubrication. On the right a full film separates the surfaces and friction rises again, slowly, because the oil itself has to be sheared.
Why this matters for modern engines
Lower-viscosity oils reduce the shearing losses on the right-hand side of the curve, which is why grades such as 0W-20 have become common. The price is that more of the engine spends more of its time towards the left, where the film is thin. Stop-start systems and downsized, highly loaded engines push in the same direction.
That is the part of the curve where additives do their work. Anti-wear chemistry builds a film that takes the contact instead of the metal. Friction modifiers lower the friction of that contact. Neither changes the viscosity of the oil; they change what happens when viscosity is not enough.
A thinner oil moves the engine along the curve. Surface chemistry decides how well it copes when it gets there.