Tech Tips

Semi-Synthetic Oils

Not all engine oils are created equal, and the choice of engine oil is very dependant upon the specific conditions under which your aircraft is operated. What may be a good choice for one operator may not be the best choice for another, even if they are operating the same model of aircraft out of the same airport. Conditions such as manufacturer’s recommendations, environmental conditions (temperatures), flight duration, aircraft loading and frequency of operation should all be considered when choosing the correct oil for your particular operation.

During teardown for overhaul, Aero Recip has often found excessive grey sludge and sometimes evidence of corrosion in engines which have been operated with semi-synthetic oil. In a letter from a well known oil manufacturer, the following statement was made:
“It is known that synthetic base fluids do not control lead deposits as well as mineral base fluids”.

Lead residue in engine oil consists of materials referred to as lead salts and acids. Residue from these materials can be left behind on polished steel engine parts and result in corrosion. Engine parts which can be particularly susceptible to corrosion are camshafts and tappet bodies.

Lead salts and acids can also be particularly harmful to aluminum engine parts. Aero Recip has often found pistons which have lead deposits and in some cases, these deposits are heavy enough to result in stuck piston rings. Interestingly enough, these lead deposits on pistons invariably grow into a heavy encrustation when the pistons are removed from the engine and placed in the open air on an engine tray for several days.

In some cases, operators are able to utilize semi-synthetic oils and avoid the the aforementioned problems. Often this is in situations involving larger higher horsepower engines which are flown frequently on trips which are long enough to thoroughly heat soak the engine and vaporize oil contamination and expel it from the engine breather.



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