1 hand-twist method
We put the lubricant between the thumb and index finger and twist repeatedly, The good lubricating oil may give us feel of dust less, friction-free, if they feel the sand between your fingers like a larger sense of friction, it indicates that it has impurities, can not be reused, we should be replaced with new oil.
2 light method
On a clear day, we can sprinkle the oil with a screwdriver, and put a 45 degree angle with the horizontal. In the light, we can clearly see no debris in the lubricant, it can continue to use, if it has too much debris, please replace the oil.
3. Droplet trace method
Take a clean white filter strip, drip a few drops in the filter strip,when the oil leakage, if the surface have black powder, and have a sense of resistance when we touch it by fingers, then the oil may have a lot of impurities, a good oil is not have powder, we may feels dry and smooth, and it appears yellow marks.