Aviation Piston Engine Oils - FAQs
What is a Monograde Oil?
Monograde oils are designed to be used within a defined temperature range. The monograde oils are classified by the thickness (or viscosity) of the oil. The intention is to ensure that the oil will cycle as required under specific operating conditions. In cold climates a thinner grade, e.g. SAE 30, is more likely to be required whilst in warmer climates a thicker grade e.g. SAE 100, is more likely to be selected. They can also be characterized by the presence of an additive package ("AD" or ashless dispersant oils) or the absence of such performance additives ("S" or "straight" oils).
What is a Multigrade Oil?
Multigrade oils are designed to provide a more consistent viscosity across a wide range of temperatures. They are usually formulated around premium quality base stock (mineral and/or synthetic blend) enhanced with an additive package.
What is Viscosity?
Viscosity is a measure of an oil's resistance to flow at a given temperature. In order to meet the requirements of different OEMs and operating conditions monograde oils are available in a range of viscosities or weights. The full range of Castrol Aviator grades is:
SAE 30 / 65 weight
SAE 40 / 80 weight
SAE 50 / 100 weight
SAE 60 / 120 weight
What is Weight?
Weight is the viscosity in SUS (Saybolt Universal Seconds) measured at 210 degF.
What is SAE?
The Society of Automotive Engineers is an international non-profit educational and scientific organisation dedicated to advancing mobility technology. They develop technical information on all forms of self-propelled vehicles including aircraft and disseminate this information through many channels, including oil specifications.
What specifications are used to qualify piston engine oils?
The two SAE specifications used are:
SAE J1899 (formerly MIL-L-22851) - Ashless Dispersant oils
SAE J1966 (formerly MIL-L-6082) - Straight grade mineral oils
The military specifications were superseded by the SAE specifications in the late 1990s. Reference is likely to be made to the military specifications for many years to come as they are embedded in the industry.
Qualification against one of these specifications is only achieved after an extensive test programme overseen by the US Navy. This begins with laboratory testing, progresses to a 150 hour engine test before finally being evaluated on wing.
All Castrol piston engine oils meet these specifications The Castrol Aviator S straight grades meet the SAE J1966 specification. The Castrol Aviator AD Ashless Dispersant grades and the Multigrade meet the SAE J1899 specification.
What is a Straight grade oil?
Straight monograde oils are designed to be used when breaking-in a new or recently overhauled engine. They are formulated from mineral base stocks, typically further enhanced by a low concentration of anti-oxidant and pour point depressant for improved low temperature performance. These oils are also referred to as Running-In or Break-In oils. They are designed and formulated to provide the correct level of lubricant breakdown and controlled cylinder wear to help lap and seal the piston rings.
Straight monograde piston engine oils are approved against SAE J1966 specification (superseded MIL-L-6082E).
Some engines use these oils beyond break-in so if in doubt please refer to your engine manual/manufacturer.
What is an Ashless Dispersant oil?
Ashless Dispersant oils are formulated from base stocks blended with additives designed with a range of objectives which may include enhancing low temperature fluidity, high temperature stability, corrosion inhibition and anti-wear protection. The additive system is ashless and of a dispersant nature offering greater engine cleanliness.
Ashless means that the product does not contain any metallic components - this is important because it reduces the formation of harmful metallic ash deposits within the engine. Dispersant means it will hold small particles in suspension if they do not dissolve, allowing these particles to be carried away from critical areas and filtered out. This helps keep the engine clean.
Ashless Dispersant piston engine oils are approved against SAE J1899 specification (superseding MIL-L-22851D).
Are all Piston Engine Oils the Same?
No. Meeting the requirements of the respective SAE specification is only the beginning. Each manufacturer uses different additives to formulate their products. For example the Castrol Aviator AD grades not only exceed the requirements of SAE J1899, but uniquely all grades include the load-carrying/anti-scuff additive required for certain Lycoming engines and beneficial to most.
Can I mix Castrol Aviator S grades with Castrol Aviator AD grades?
Yes, all approved SAE J1899 and SAE J1966 oils should be compatible. The most common reason for needing to mix oils is following the replacement of a cylinder on a mid-time engine which has been running on a Castrol Aviator AD grade. Following the cylinder replacement it is normal to switch to a straight Castrol Aviator S grade for one or two changes to break in the new cylinder. Once the rings are properly seated you can switch back to Castrol Aviator AD.
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