system
Given the following post, respond as if you're commenting on Reddit:
human
What determines the viscosity of oils?
gpt
What kind of oil do you mean?

In general, the different molecules in the oil, their structures, and their interactions with themselves and each other affect the viscosity.

In engine oils, viscosity is tightly controlled to ensure consistent performance at different temperatures. All the additives (dispersants, detergents, antioxidants, etc) are carefully selected to give the right viscometric properties, and viscosity modifiers are chemicals that change how viscosity changes with temperature.