At what temperature would an SAE 40 and a 5W-40 have the same viscosity?

An SAE 40 and a 5W-40 motor oil have different viscosity characteristics, particularly at lower temperatures. The "SAE 40" designation indicates that the oil has a viscosity rating of 40 at 100°C, while the "5W-40" indicates that it has a viscosity rating of 5 when cold (the "W" stands for winter) and a viscosity of 40 at 100°C as well.

At higher temperatures, such as 100°C, both oils will exhibit the same viscosity. This means that at this specific temperature, both SAE 40 and 5W-40 will perform the same in terms of flow and lubrication, despite their differing performance at lower temperatures. The 5W-40 oil is specifically designed to remain fluid at lower temperatures, which is beneficial for cold starts, while maintaining the same viscosity as the SAE 40 when the engine is at operating temperature.

In contrast, temperatures of 50°C, 150°C, and 200°C would not provide the same viscosity for both types of oil, since the 5W-40 would have higher viscosity at lower temperatures (like 50°C) when compared to the straight SAE 40. Conversely, as temperatures increase