First, smell method: draw out the oil dipstick close to the nose and smell it. If there is a strong sour smell, it means that the oil of the cleaning suction truck has deteriorated and should be replaced.
Second, hand twisting method: grind the old engine oil taken out from cleaning suction truck engine with thumb and index finger repeatedly. The good hand feel has lubricity, less wear debris and no friction. If you feel impurities, poor viscosity or even astringency, you should replace it.
Third, color identification method: take a clean white filter paper (a better towel paper can be used), drop a few drops of old oil on the paper, after the oil of the cleaning suction truck leaks, the oil with good quality is free of powder, dry and smooth by hand, and the Yellow soakage belt is clear. It is dark brown and has impurities. It should be replaced.
Fourth, light method: take out the oil dipstick of the cleaning suction truck and lift it up to 45 degrees. Observe the oil drop under the light, and it is good to see that there is no debris in the oil drop. If there is too much debris, it should be replaced.