When a straight mineral oil turns dark or black, it usually means that the oil is starting to oxidize and needs to be changed. Because mineral oil doesn’t absorb much of the dirt and sludge in your engine, the oil stays clean and the inside of your engine gets dirty. Ashless dispersant oils, on the other hand, are designed to get dirty so that the engine will stay clean. 

Just how quickly the oil turns black depends on a number of factors, including the condition of the engine, the dirt load, the oil temperature, the normal air/fuel mixture, the type of fuel, the time since the last service, and the frequency and duration of your flights. Basically, the important thing to remember is to change your ashless dispersant oil on calendar and engine time, not according to its color. Also, oil analysis can help ensure that the oil is still in good condition even though it may have turned black.

In most cases, the multigrade oil will run cooler. For a hot-running engine, like turbocharged, high performance or aerobatic aircraft engines, this is good, but for a cool-running engine it can be a disadvantage. If the engine runs too cool, it can't boil off excess moisture and unburned fuel, so there can be a tendency to form acid buildup. For cooler-running engines, pilots should use a winterizing kit, or check with their mechanics on how to keep oil temperature up.

Yes. The thickness, or viscosity, of an oil is directly affected by the temperature. Therefore, if an engine's oil temperature is increased, there will be a small, but proportional, drop in the oil pressure as well.

A number of pilots have asked this question. The answer is a definite no. When Shell first started evaluating multigrade aviation piston-engine oils over 25 years ago, testing proved that multigrades formulated only with mineral base oils did not have adequate base oil viscosity (thickness) to properly lubricate all high load points in the engine. Then we tested and flight evaluated a formulation made with all-synthetic base oils.

This formulation had excellent antiwear characteristics in all tests run. However, in the flight evaluations, some engines would reach 600 to 900 hours, then lose oil consumption control and/or compression. When the engines were disassembled, we found that the piston rings were covered with a gray tacky substance that was primarily made up of the lead by-products of combustion (from the use of leaded aviation gasoline).

Although synthetics are excellent lubricants with good high temperature stability and very good low temperature flow characteristics, they are relatively poor solvents.

In an aircraft engine, the lead by-products of combustion must be dissolved by the base oil so they can be carried away from the ring belt area and removed from the engine when the oil is changed. Anticorrosion, antiwear AeroShell Oil W 15W-50 is formulated with 50% synthetic base oils to give it the excellent low temperature flow needed for quick lubrication during cold starting. 

The synthetic base oils, along with the unique antiwear additive system, give it antiwear protection unequaled by any other product on the market. In addition, its mineral base oils provide lead absorbency to guard against ring sticking and excessive sludge. The bottom line: The synthetic component of AeroShell Oil W 15W-50 will not harm your engine. Instead, it gives you the best of both oils.