1. Light Inspection Method

On a clear day, use a screwdriver to lift a drop of lubricating oil at a 45-degree angle to the horizontal plane. Hold it up to sunlight and observe the oil droplet. Under illumination, the absence of metal swarf in the lubricant indicates satisfactory condition and continued usability. Should excessive swarf be present, replace the lubricant.

2. Oil Residue Test

Place several drops of oil onto clean white filter paper. Once the oil has permeated the paper, examine the surface. If black powder appears and feels gritty to the touch, this indicates significant impurities in the lubricant. Good lubricant leaves no powder residue; it feels dry and smooth to the touch, leaving a yellowish mark.

3. Oil Flow Observation Method

Take two measuring cups: one containing the lubricant to be tested, the other empty on the table. Raise the cup with lubricant 30-40 centimetres above the table and tilt it, allowing the oil to flow slowly into the empty cup. Observe its flow characteristics: high-quality lubricant should flow in a thin, even, continuous stream. If the flow fluctuates in speed or forms large clumps, the oil has deteriorated.

4. Thumb-and-Finger Method

Rub the lubricating oil repeatedly between your thumb and index finger. Good lubricating oil should feel lubricating, produce minimal abrasive particles, and exhibit no friction. If you perceive significant friction akin to sand between your fingers, this indicates the oil contains excessive impurities and should be replaced with fresh lubricating oil.