With the correct oil, friction losses in an engine are reduced to a minimum. This is done by taking into consideration circumstances as engine usage, ambient temperature, time of year and climate, location and engine design. The engine manufacturer usually recommends a certain type of oil to use taking all of these circumstances into account.
Lubrication plays an important part in the life of the engine and during maintenance it will be replaced, on certified aircraft the pilot can only replenish it. Without it the engine would fail within minutes, keeping a watchful eye during flight is therefore important.
As oil is used to reduce friction and wear it eventually picks up metals from the engine, measured in parts per million (ppm). Through analysis we can determine how much and which type. If the concentration of a certain metal rises its a good indication that wear is increasing and maintenance action might be needed before the engine fails.
Spectrographic analysis is a popular way of identifying wear characteristics of an engine, it is adopted by the military, commercial and general aviation. As engines are designed with various metals and alloys, the system provides oil under pressure or splashes oil to the areas needed and subject to friction. During the normal course of operation these parts undergo minimal and minute wear. Submicroscopic material is released and suspended in the oil. Analysis identifies these metals and their amounts in parts per million (ppm) and gives a good view of the engine internals regarding wear during its operation.
There are two ways of analysis: atomic absorbtion and atomic emissions. Both will identify submicroscopic particles in the oil in parts per million (ppm). But with atomic absorption particles smaller than 5 micron will be detected and with atomic emission particles smaller than 10 micron are detected. Either method is good, but they should not be used together on the same engine. Comparing these different reports will then have little meaning.
It must be said that all the oil analysis of your engine should be carried out by the same lab every time (as to assure the same work procedures), and even an one time analysis of a batch of fresh engine oil should be done to set up a base line for the engine. It goes without saying that the engine should be run on the same type and brand of the oil for the results of multiple analysis to have any meaning.
Oil analysis is an extra tool which can help identify problems in the engine before they develop into threatening issues during flight. It gives the technician insight information in the normal wear of the engine and any deviations from the normal trend should be investigated.
The frequency of aircraft / engine use is one of the prime factors in determining how the oil will perform and how often it should be changed. Frequently flown aircraft, think one hour a week and regular (50 hour interval) changes will make any oil look good. This behavior keeps the oil covering all internal parts. There is some debate that oil will 'run off' engine parts after a while, but oil will always stick to metal and keep it covered. This, however, could not be the case of piston oil compression and scraper rings as they are subject to high temperatures and oil does tend to get burned off, although this is only in minute quantities.
Engines flown less than 100 hours per year are good candidates for corrosion formation (my personal opinion is that in this day of age engine corrosion can be properly taken care of by good metallurgy and modern engine oil and I wonder why the established engine manufacturers do not apply modern alloys in their engines to combat corrosion).
Between aircraft use, engine oil should maintain a coating on all internal parts, if not, the surfaces will begin to oxidate within a short period of time. If left unattended longer, the oxidation will damage the steel parts of the engine. It is clear that safety will be an issue here.
Frequent oil and filter changes is a good way to minimize these effects. Thicker oil would help too as it 'sticks' better to the metal. But this has the disadvantage that it takes a couple of seconds for the oil to be up to pressure and reaching all parts moving, especially in winter. Hence the need for multigrade oils in which we have 15W50 and 20W50 which are thinner at lower temperatures facilitating quicker oil pressure but are able to be 'thick' enough at engine operating temperatures.
Moisture is formed when the engine oil cools and water condenses. Regular flying with oil temperatures reaching 100°C will make sure that all water is boiled off. Ground running is just not enough. Its too short for all parts to get up to operating temperatures and in the end will do more harm than good. It increases water formation and corrosive attack.
Engine combustion byproducts are picked up by the oil and will form, when mixed with condensation, acids capable of etching into the metals of the engine. Resulting in more corrosion. Frequent oil changes, even on a four monthly basis when not flying frequently (time limited as opposed to hour limited), will help against acid formation.
The location where the aircraft is used or parked, coastal and or high humidity places, will contribute to corrosion. As said above, if flying infrequently your are in said locations, do more changes to minimize possible corrosion and this will help in keeping the engine in good health.
Oil and filter should be changed regularly but the use of the aircraft (or the lack thereof) and other factors dictates if the oil must be changed sooner than prescribed by the manufacturer to prevent any corrosion formation in the engine. Oil analysis is a great tool to see if the oil is up to the task its designed for in your engine and particular use of the aircraft.