The difference between single-grade and multi-grade oils is mainly seen in their viscosity when driving in varied conditions. Multi-grade engine oils are formulated to have two different flows depending on driving situations. One flow is for cold weather and the other one when the engine is warm. Generally, multi-grade oils start operating as thinner oils and then get thinners once the engine is warm. They are essential for protecting the engine in cold temperatures. Single-grade oil, also known as mono-grade, are usually thin oils and are suitable when cold starting in very low temperatures like winter.