Determining whether engine oil has deteriorated can be comprehensively assessed through multi-dimensional methods. First, observe the colour and state: normal engine oil is translucent yellowish-brown; if it turns black or the dipstick scale is difficult to discern, it may have oxidized. When tested with slow qualitative test paper, if a four-level sediment ring or a dark brown diffusion ring appears after 24 hours, the oil needs to be replaced. Second, smell the odor: fresh engine oil has a slight fragrance; if a pungent sour smell occurs, it indicates that the additives have failed or the oil has degraded. Additionally, observe the oil flow state: high-quality engine oil should form a long, thin and uniform filament when poured; if intermittent dripping occurs, the fluidity has decreased. For diesel engines, attention should also be paid to emulsification; if the oil shows foam or milky stratification, it indicates water contamination. In daily driving, if symptoms like sluggish acceleration, idle vibration or increased noise are observed (after ruling out other faults), these may also relate to deteriorated engine oil performance. It is best to replace mineral oil every 5,000 kilometres or 6 months, semi-synthetic oil every 7,500 kilometres or 9 months, and fully synthetic oil every 15,000 kilometres or 12 months at most. However, for vehicles subjected to long-term short-distance driving, high-temperature or dusty environments, the interval should be shortened by 30%. Turbocharged models or high-performance engines require inspection 20% earlier than the standard mileage interval. If a vehicle remains parked for over 3 months, replacing the oil is recommended even if the mileage threshold hasn't been reached, to ensure engine protection.
