Viscosity is one of the engine oil's most important characteristics. Viscosity is defined as a liquid's resistance to flow. The two most commonly used aviation oils are 80 and 100 weight. These numbers are exactly twice the more generally recognized SAE (Society of Automotive Engineers) ratings of SAE 40 and SAE 50.
In recent years multi-weight aviation oil has become popular. The theory behind multi-weight oil is that it will act like a low viscosity oil at cold temperatures and high viscosity oil at high temperatures. Multi-viscosity automotive oil has been available for years, but only recently has it been generally accepted for aircraft use.
You have three basic types of aircraft oil - mineral, ashless dispersant, and synthetic. Mineral oil is the oldest of the three and has been used in engines since the beginning of powered flight. Its primary disadvantage is its tendency to oxidize when aerated at elevated temperatures. Today, mineral oil is primarily used to break-in new and newly overhauled engines. Because it lacks the additives used in ashless dispersant oil, mineral oil is less slippery and allows for faster and more complete seating of piston rings and other internal engine parts during the break-in phase.
Ashless-dispersant oil has dominated the piston oil scene for some time. This oil has additives that accomplish a number of things. The additives prevent the formation of ash deposits and the dispersant characteristics keep contaminants from clumping together. By keeping the contaminants in suspension, the oil can be filtered better and the remaining suspended contaminants can be flushed from the engine when the oil is changed.
Synthetic aircraft engine oil is relatively new and is not as yet universally accepted. It seems to do a better job in the cleaning department, and synthetic oil's lubrication characteristics are nearly the same as ashless-dispersant oil. Synthetic oil can soften rubber products and resins that are found in paper-based oil filters.
Federal Aviation Regulation Part 43 allows private pitots to perform preventive maintenance. Changing the engine oil and oil filter (if any) and cleaning the oil screen is one of the most important preventive maintenance items for your engine.
The first step is to determine what brand and type of oil to refill the engine with, and what brand and type of oil filter, if any, to replace. Many pilots are surprised to learn that many aircraft engines don't have an oil filter. Unfortunately, the filter and the adapter to hold it are an extra cost, optional item on some engines.
Not having an oil filter is poor economy because, in the long term, it saves time and money in oil changes, maintenance time, engine life, and reliability. Generally, an engine with an oil filter requires an oil and filter change every 50 hours of engine operation. An engine without a filter requires an oil change every 25 hours. All aircraft engines, regardless of their filter status, have an engine oil screen. It must be removed, inspected, and cleaned at every oil change.
After ensuring that you have the proper oil, filter (if any), and oil screen gasket on hand, remove the engine cowling and inspect the engine for obvious defects and oil leaks. Even if the aircraft has a quick drain that enables you to change the oil without removing the cowling, it's a good idea to uncover the engine and inspect it because it's a good way to find possible problems before they become serious.
Before draining the oil, run the engine to warm the oil and the metal engine components. Warm oil drains faster and more completely and carries away more solid contaminants from the bottom of the oil sump. Remove the oil drain plug and let it run freely into a container large enough to hold the entire engine oil supply. Aircraft engines commonly hold 8 to 12 quarts of oil.
Next remove the oil filter and cut it open. Spread out the filtering element (usually paper) and inspect it for metal particles. Then remove the oil screen, inspect it for solid particles, and clean it.
After the oil has stopped draining, replace the drain plug and safety wire to prevent engine vibration from loosening it. Replace the oil screen using a new gasket, and safety wire it to secure it from vibration. Then install the new oil filter with a new rubber gasket, and lubricate the gasket with clean engine oil before you tighten the filter to the proper torque. You safety wire the filter, too.
Fill the engine with the proper amount of new oil, and take into account the amount of oil the new filter will hold. Next, run the engine for a brief period, and pay special attention to the oil pressure gauge. Shut the engine down and inspect it for oil leaks, especially around the new oil filter, the oil screen, and the oil drain plug. Wash down the engine, engine compartment, and engine cowling, and then replace the cowling. The final step is to make the proper entry in the aircraft engine maintenance records to indicate the work performed and the parts used.
During a preflight inspection pilots should look for several oil related items. Be sure that the oil level is within the manufacturer's recommended limits. If the engine has just been running, wait a bit before checking the oil. The engine compartment and the oil dipstick will be quite hot, and a brief cool-off period will prevent burns. Also, it takes a while for the oil to drain down through the engine to the sump following engine shut down. If you fill the oil level to the full mark immediately after shut down, you could overfill the engine.
Inspect the engine compartment and what you can see inside the cowling for signs of oil leaks. If you find any observable amount of oil in the engine compartment, have a mechanic examine the aircraft before flying it.
The lubrication systems of today's aircraft engines and the modern aircraft engine oil and filters we use in them have evolved into reliable and effective products. With a little understanding and a few basic precautions, both will give long and worry-free service.
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