The difference between 10w-30 and 5w-30 oil is the thickness of the oil during cold weather conditions, 5w-30 being the thinner of the two and therefore recommended for colder operational climates. Engine oil is rated on a viscosity scale by the Society of Automotive Engineers, which determines its cold- and warm-weather rating; the cold rating is indicated by a “w” for winter.
Automotive oil manufacturers currently produce multigrade oil, which means it behaves differently at different temperatures. The Society of Automotive Engineers grades oil based on its performance at different temperatures by measuring the amount of time it takes to flow through a length of tube. Oil that takes 30 seconds earns a grade of SAE 30. Lighter-weight oil passes more rapidly through the same tube but thickens under colder temperatures, allowing for better performance. Therefore, an SAE 10w-30 performs like grade-30 oil during warm temperatures and grade-10w oil in colder climates.
In the past, oil manufacturers only offered single-grade oil that needed to be changed for seasonal operations. Heavier oil that offered good performance in the summer would have to be changed for lighter-weight oil for easier starting in the winter. A single-grade oil has one number to signify its viscosity, such as SAE 50 or SAE 10w.