Hi Johnny,
For aviation or aviation derived gas turbines requiring POE synthetics, there are two basic types of oil, each with some sub-categories.
MIL-PRF-7808 (military spec) oils are designed for lower temperature environments such as arctic/Antarctic military bases. These oils are often referred to as "3 Centistoke" or 3 cSt oils because of their viscosity at 100C, but more important is their viscosity at -51C for cold starting. 7808 oils are used mainly by militaries, US (Air Force) and many foreign as well. They are sometimes also used in commercial APUs which have to re-start at altitude.
The other and larger type is MIL-PRF-23699 oils or "5 Centistoke" (5 cSt) oils. These have a starting viscosity of -40C and are used by the US Navy, Army and other branches, as well as foreign. The US Military uses about twice as much 23699 as they do 7808, but the biggest users by far are commercial airlines where they are used exclusively.
Most military buyers purchase against the appropriate military specs only, except some (Europe) who approve by brand. Commercial airlines, however, also require approvals for each engine model from the engine builders, and this process costs millions and takes up to ten years to complete the various bench and flight testing. For this reason, there are only three serious marketers to the airlines - Air BP (was Exxon), ExxonMobil (was Mobil), and Shell.
The 7808 oil has two sub-categories, 3 cSt and 4 cSt, but the 3 cSt dominates and both are scheduled to be replaced in a few years with a new specification for 5 cSt. Turns out the AF discovered it does not really get cold enough at their worse bases to require 3 cSt oils, so they will move to a 5 cSt oil and hopefully will have a unified spec with the rest of the armed forces.
The 23699 type has three sub-categories called STD (standard), C/I (corrosion inhibited, i.e. same as STD but with rust protection), and HTS (high thermal stability, designed for hot problem and future engines). The US Armed Forces use mostly the C/I type, but small quantities of STD and HTS are also used for special circumstances.
Commercial oils are mostly of the STD and HTS type plus OEM approvals. They don't have the rust problems that the Navy suffers, and have more hot running engines needing HTS than the military (more concern with fuel efficiency, long hauls, and long OCIs).
A new specification was issued by the SAE last year intended for commercial use called AS5780. This spec encompasses the 23699 spec but adds more tests required by commercial engine builders. Additional engine and flight evaluations, however, are still required before use. This spec has two sub-categories, SPC (standard performance capability) and HPC (high performance capability) corresponding closely with the military STD and HTS classes.
Helicopter and turboprop engines operate under higher loads due to the gears and usually need a different oil. Most helicopters work fine with the standard 23699 oils, but those needing more wear protection use a MIL-PRF-85734 oil, also known as a “DoD” (department of defense) oil. These oils are essentially the same as the 23699 oils except they contain a small dose of an EP additive to boost load carrying. Since this additive tends to eat certain seals, these oils have their own spec as opposed to just a sub-category under 23699. Turboprop turbines generally use a heavier oil, 7.5 cSt, to cope with the higher loads from gear reduction.
The formulations used for military spec oils and commercial oils are very similar and most military approved oils would work quite fine in commercial operation as well. The difference is commercial oils must have higher proof of their worthiness, especially since commercial operation is different than military, so engine builder approval is required. The FAA considers the oil to be part of the engine and imposes responsibility on the builders.
There are differences among the major brands of commercial oils, especially with respect to deposits. The Air BP 2197 (HPC) is generally considered to be the cleanest oil in the field and the target for all new development. Among the SPC oils there are differences in deposit tendencies, load carrying, seal compatibility, approvals, and cost, so airlines have to consider all properties for their fleets to determine best value.
Sorry for the long post – probably more than you wanted to know but sometimes it’s hard to stop when I get started.
Tom
, so I thought I would start one.
