Base stock(s), 78 percent: This is the part that is "just oil," whether petroleum or synthetic.
Viscosity index improver, 11 percent: This additive makes an oil "multigrade" by slowing the rate at which it loses viscosity as its temperature rises.
Ashless dispersant, 6 percent: AD molecules surround particles of varnish or gum, preventing them from clumping together or sticking to parts surfaces, such that they are carried to the oil filter where they are removed.
Metal detergent, 3 percent: Similar to above, often with ability to neutralize acids.
Inhibitor: anti-rust, anti-corrosion.
Anti-wear, such as ZDDP: Forms a solid lubricant layer on points of contact, preventing scoring and other surface damage.
Pour point depressant: At low temperature, prevents formation of wax-crystal networks that can solidify the oil.
Friction modifiers: Form adsorbed layers of long-chain oil-like molecules on parts' surfaces to make them more "slippery."
Anti-foams: Speed air release by reducing the surface tension of air bubbles.