Why use them and what are the differences?
In order to answer this question, we first need to look at what an oil does and how it does it.
Oil has many functions to perform in the engine, but the primary one is to reduce friction between moving parts by separating moving surfaces with a layer of oil. This oil must not only separate the two surfaces, but must also support any load that is being applied between them, so that the load can be transferred from one surface to the other.
If the oil did not support this applied load, then the two surfaces would force through the oil film until they came into contact, potentially causing significant and irreparable damage.
An important indication of how much load an oil can carry is the measurement of the oil's viscosity.
Viscosity is the resistance to flow of a particular fluid. Under the same conditions a liquid with a low viscosity, such as water, will flow more quickly than a liquid with a high viscosity, such as syrup. However, in general, an oil with a high viscosity will support more load than one with a lower viscosity.
There is an obvious balance to be struck here between having the oil viscous enough to support the required amount of load whilst also being fluid enough to pump effectively.
The problem is further compounded by the fact that as an oil is heated, the viscosity drops and vice versa. This relationship between temperature and viscosity is known as the viscosity index; the higher the viscosity index of an oil, the less it's viscosity changes with temperature.
The ideal situation is to have an oil who's viscosity doesn't change with temperature, so that it maintains the same load carrying ability as well as the ability to be pumped at all temperatures. Unfortunately this is impossible to achieve, so we are left with certain compromises.
The majority of the aviation piston engine oils on the market are mineral monogrades (both the straight and monograde ashless dispersant or 'W' oils). The viscosity index of these mineral oils are fairly uniform relatively unimpressive due to the very nature of the base oils used.
This means that whilst a higher grade oil (e.g. 100) oil may be satisfactory for operation during warm weather, it may well become too thick to pump effectively at the lower temperatures experienced at start up. Conversely, if a lower grade oil (e.g. 80) is used during hot weather, it may not prove to be viscous enough to support the loads required when at the elevated operating temperatures.
This is why many engine manufacturers advise that the oil grade is changed as climatic temperatures change. Note: you should always refer to your engine manufacturer's recommendations for clarification on which grade to use at which temperature.
The ideal solution to this is to produce an oil which has a higher viscosity index (i.e. it's viscosity changes less with temperature). In this way in cold weather it will pump effectively, but still support high load at high temperature.
This is the concept of the multigrade oil and there are two principle ways of achieving these objectives:
Mineral multigrades use a light weight mineral oil (the same as a light weight monograde oil), but include an additive called a Viscosity Index Improver. The best way to visualise this viscosity index improver is as a long chain molecule which curl up like a ball of string when cold, but then uncurl as the temperature increases.
Thus when an oil is cold, the presence of the viscosity index improver has very little effect and the oil flows well as the base oil is a low viscosity oil. As the oil heats up, the viscosity index improver uncurls with the effect that it tends to restrict the motion of the oil, or 'thickens' it, which to some extent counteracts the decrease in viscosity of the base oil. This enables the oil to support more load than would otherwise be possible.
However the viscosity of an oil which contains a viscosity index improver depends on the rate is made to flow (or sheared). It may decrease rapidly if the oil is sheared rapidly, and this decrease can be temporary or permanent.
A temporary loss in viscosity develops when high shear rates (which frequently occurs in engines when one surface moves quickly past another) force the large viscosity index improver molecules to align themselves in the direction of flow. More seriously, a permanent loss of viscosity may occur if the shear rate is sufficient to physically break the large molecules into smaller units. This can happen in oil pumps and the like. Both of these scenarios reduce the oil's viscosity and therefore the load carrying ability.
The vast majority of 20W-50 aviation multigrades on the market are mineral multigrades.
Semi-Synthetic oils use a blend of mineral oil and a synthetic hydrocarbon oil.
Due to the naturally high viscosity index of the synthetic oil - it's viscosity changes less with temperature when compared to mineral oils - there is no need to add a viscosity index improver.
Another advantage of using a semi-synthetic oil is that the synthetic component of the oil has a higher thermal stability and therefore degrades at a slower rate than mineral oils. This leads to the oil both performing as an effective lubricant for longer and also producing less acidic compounds, the byproducts of oil degradation, which in turn reduces the risk of acid attack in the engine.
AeroShell Oil W 15W-50 is virtually the only semi-synthetic aviation mutigrade on the market at present, and has given years of excellent performance throughout the world.
The obvious question is, "Why not produce a fully synthetic oil if it so good?".
The answer is simply that, unlike automotive engines, aviation engines run on 100LL (Avgas) which contains a much higher concentration of Lead than ordinary 4 star automotive fuel. The combustion of this fuel inevitably leads to Lead getting into the oil in the crankcase where it could form Lead deposits, and may lead to subsequent failure. However if the oil has a mineral oil content to it (either a fully mineral oil, or a semi-synthetic) then the Lead is dissolved by the oil, whereas a fully synthetic oil does not have the capacity to do this.
One of the advantages of using AeroShell Oil W 15W-50 comes when the aircraft is not flown frequently as the oil contains both a corrosion inhibitor and an anti scuffing additive (LW16702) to help the occasional flyer.
If the aircraft cannot be flown with the frequency required to keep the oil 'dry' (a minimum of 1/2 hour cruise every 2 weeks), the corrosion inhibitor will suppress the formation of any corrosion during periods of inactivity, which would otherwise form due to the action of acids and water.
Furthermore, once the aircraft engine is started up after being inactive, the anti scuffing additive will have coated all the internal metallic surfaces with a molecular layer so that metal to metal contact is prevented if there is no oil present. This is particularly important during the first few seconds after start up as the oil pump will not pump oil to all the extremities of the engine immediately.
Again not all oils contain these additives, but combined with the natural advantages of using a semi-synthetic oil, we believe that AeroShell Oil W 15W-50 represents the premium quality choice in the aviation piston engine oil market.