Test for minute particles of copper
Take some of the residue engine oil out into the sun and look at it with the sun shining on the oil. Look for any sparkles in the oil. Your inspection has be-gone!
This seems kind of a strange request but I have found that a bushing failure can result in many microscopic sized particles suspended in the oil. Sunlight brings out the sparkles. Under artificial light you don't see the sparkles.
There is a lot you can tell from the oil using your sense of sight, smell, and touch.
Smell
Test for oil oxidation
Have you ever smelled old dirty engine oil? It stinks and has a burnt odor that is unmistakable. Just pulling the dipstick out you can smell it.
If you have never smelled dirty oil then now is the time start looking. You don't have to put your nose in it. Just be aware of any oil odor when you check the oil on the dipstick or are changing your oil. Normally you shouldn't smell anything, or with used oil a slight odor. If you rub some of the old oil between your fingertips you can better smell it.
Color
Test for oil contamination by soot and oil oxidation
When you check the oil level on the dipstick you are checking not only the oil level but also the oil's condition.
Used oil in aircraft piston engines looks like the oil in the picture above. It should be a dark golden tan. Some brands may have a slightly different color. Each time you check the oil on the dipstick, notice the oil's color. It should be clear to tan. If it is black then it is oxidized and contaminated. Different engine types produce slightly different colors, but get to know your engine. Any rapid change in color indicates a problem.
Oil on Water
Test for water in oil.
Drop a drop of oil onto water. Good oil will stay together. Contaminated and oxidized oil will spread over the water surface.
Rub the Oil
Test for particles in oil.
With a drop of oil between your fingers you can rub your fingers together. You shouldn't feel any grit or grime.
The chromatography test (blotter spot test)
Reveals insoluables in oil - oil cleanliness
This is a fancy name for a dirt-simple test (pardon the pun) that works quite well. Drop the oil on a heavy piece of white absorbant paper. Support the paper on its edges so that the oil drop doesn't contact the counter. The compounds in the oil have different absorbsion rates and will separate. Uncontaminated oil will create a simple uniform circle. Rings of differing color indicate contamination. You can also use the blotter spot test on hydraulic fluid to spot contamination.
