The recommended frequency for refilling the flu in a small car is typically every 3,000 to 5,000 miles or every three to six months, depending on driving conditions. This interval ensures that the engine remains properly lubricated and protected against wear and tear.

The reason for this frequency is based on the importance of maintaining the correct fluid level and condition for optimal engine performance and longevity. According to the Car Care Council, engine oil is crucial for reducing friction, cooling engine parts, and carrying away heat and debris. Over time, oil degrades due to heat, oxidation, and the accumulation of particles. When the oil becomes too thin or too thick, it loses its ability to protect the engine effectively. The American Petroleum Institute (API) suggests that regular oil changes are essential to prevent engine damage and maintain fuel efficiency.

Data from the National Institute for Automotive Service Excellence (ASE) indicates that oil should be changed at least every 3,000 miles for most vehicles, but this can vary based on the type of oil used and driving conditions. For synthetic oils, which are more durable and can last longer, the interval can be extended to 5,000 miles or more. Additionally, the Car Care Council reports that driving habits, such as frequent short trips or stop-and-go traffic, can accelerate oil degradation, necessitating more frequent oil changes.

In summary, the recommended frequency for refilling the flu in a small car is every 3,000 to 5,000 miles or every three to six months, depending on the type of oil and driving conditions. This practice ensures the engine remains well-lubricated and protected, contributing to its longevity and efficiency.