The one real difference
Both oils share the same 0W winter rating, so cold-start flow is effectively the same — both pump quickly when the engine is cold. The meaningful difference is hot viscosity. 0W-16 is thinner at full operating temperature, while 0W-20 holds a slightly thicker film.
0W-16 is among the newer ultra-low-viscosity grades and shows up mainly in recent economy and hybrid engines, where the thinner oil reduces internal drag and can support a small fuel-economy gain. These engines are built specifically for that low viscosity, with clearances and oil pumps designed around it. 0W-20 remains the common grade for a wide range of modern engines. The thinner oil is not automatically a compromise on protection when the engine is engineered and approved for 0W-16.
Which should you use?
Use the grade your owner’s manual specifies for your engine. This is especially important with 0W-16, because engines designed for it depend on the thinner oil; moving up to 0W-20 without manufacturer approval is not automatically safer and can run counter to the engine’s design.
Some manuals list 0W-20 as an acceptable backup for a 0W-16 engine, often only for short-term use or specific conditions. If yours does, follow exactly what it says. If only one grade is listed, use it. And remember that viscosity is separate from specification: whichever grade you choose still has to meet the oil standard or approval your manual requires.