Technical Articles - As published in The Blue

Many people have an opinion about this – some wanting to take advantage of what they see as being modern technology, and others feeling that old engine designs need old style oils. There is in fact some merit to both cases, and the key is to ask what type of engine you have, how you operate it and in what temperature environment. Let me give some background before stating the cases for multigrade’s prosecution and defence.

Mulitgrades are intended to be thin at low temperature so they circulate around the engine quickly when it is started from cold. At high temperature, multigrades do not thin out as much as single grade oils so that a 15W-50 is actually thicker than a W100 oil when it is above 100 oC – the oil temperature typically inside the engine (remember the oil temp gauge normally measures the oil temperature going into the engine, which is lower). This means that, in principal, a multigrade is able to do the same lubrication job as a single grade oil when the engine is running, but helps the engine to circulate oil better when the engine is first started.

The numbers give a clue to the performance at the different ends of the temperature spectrum. A 15W-50 will be more fluid at low temperature than a 20W-50 for example, whilst still maintaining the high temperature viscosity to help protect the engine from wear when running.

There are some other advantages to multigrades if they have a semi-synthetic composition like AeroShell’s 15W-50. Semi-sythetic oils have a better thermal stability, meaning they can withstand higher temperatures, and also tend to lower oil temperatures by about 5 – 10 deg F when compared to pure mineral oils. Both of these features can be an advantage in engines where high heat input to the oil is a concern; for example aerobatic and turbocharged engines.

The Prosecution – The common arguments against Multigrades

Some people feel that the oil is too thin. It is apparent that the a 15W-50 is much less viscous than a W100 when pouring it into the engine, but this is the whole point as it helps the oil circulate. At normal operating temperature the two  oils look much the same.

If the engine is sat idle for long periods, then won’t a thinner multigrade will run off the engine parts more than a thicker monograde?

Whilst this can be a factor, it is not to the extent that you may think. Most of the run off happens when the oil is still hot immediately after shut-down as both single and multigrade oils are much more fluid then. Periods of inactivity are actually a problem whichever oil you use so, if you have laid your engine up for a while – say a month or more – then one tip is to pre-oil the engine before start. This can be done with a pre-oiler kit, which allows you to pump oil from the sump prior to engine start, or alternatively you can improvise. Remove the engine cowling and remove a set of spark plugs (the bottom ones would be better). Set the magnetos to ‘Off’ (if this is possible), turn the fuel off and set the mixture to idle cut off (fully lean). Then turning the motor over on the starter motor should help to pre-oil the engine (be cartful to note any restrictions in the Pilot’s Operating Handbook on the maximum time allowed for operating the starter). Removing the spark plugs allows the engine to turn over much quicker than normal as there is no compression to work against. This allows the oil pump to rotate relatively quickly and supply oil to the galleries in the engine – in other words pre-oiling the engine. Wipe up any mess from the plug holes, re-fit the plugs and cowl and start as normal.

Can Radial and inverted engines use multigrade?

Generally the safe bet is no. The main reason is that the downward pointing cylinders tend to collect oil when the engine is not in use and this can cause a phenomenon called hydraulic lock when the engine is started. When this happens, the oil partially fills the combustion space and, being incompressible, increases the pressure in the cylinder enormously when the piston moves on the compression stroke and can result in severe engine damage. It is an attempt to remove this oil that radial and inverted engine owners pull the propeller through prior to start. Obviously if an oil is more fluid at ambient temperature, then this problem is more likely to happen, so multigrades are generally not used in this type of engine. There are exceptions, such as small radials in arctic conditions, or Gypsy Major engines, which have been shown to work once they have a modified oil control ring etc., but generally these engines tend to use monograde oils. If in doubt – seek advice from your engineer, or from Shell Aviation’s web site question line.

The Defense – Understand the objectives and the options.

The real trick is to ensure that you get oil supply to the engine as quickly as possible, otherwise you risk internal damage. Even with some oil present, thin oil films give rise to higher wear rates than with a normal oil supply. This is why it is generally accepted that engine starting is when most wear occurs – some estimate up to 80%. This is why careful choice of the oil’s viscosity is significant, especially when operating in low temperatures. Oil’s viscosity increases exponentially with decreasing temperature – in other words all oils gets much thicker for every degree of temperature drop at low temperature when compared to high temperature. To illustrate this, let’s look at a single grade oil such as W100. Between 100 oC and 110 oC the oil changes viscosity by about 4 cSt, but between –5 oC and +5 oC the viscosity changes by 7,000 cSt. This is why lighter oils, such as W80 and W65, are used in colder climates and why continuing to use W100 in winter conditions can cause engine damage.

Multigrades are intended to get around this problem, but even here there are differences. The low temperature flow of a multigrade (the first number in its name) is a significant factor in determining how thick the oil is - and apparently small numbers can make large differences.

To illustrate this, in a circulation test with an engine at –10 oF (-23 oC), it took 14 seconds for 15W-50 reach the oil gallery, whereas a competitive 20W-50 multigrade took 50 seconds. With an engine running for almost a minute without any oil supply I’m sure you can imagine the potential damage - any test data (such as can be found in some of our competitors' current advertising) which ignores the effects of this crucial engine-starting phase does not tell the full story.

Whilst multigrades can be used all year round, when the weather is warm then the operator is in effect paying for performance that he is not using. Remember the relatively small change in viscosity at warm temperatures? Well this means that multigrade oils do not circulate significantly quicker than single grade oils when the weather is warm. Combine this with the fact that oils should be changed every 4-6 months regardless of engine hours, and it should be no surprise that we find a lot of customers now using AeroShell Oil W 15W-50 in the winter to take advantage of rapid oil circulation, and switching to AeroShell Oil W100 Plus in the summer, as it is less expensive. By using the W100 Plus rather than regular W100, the operator also is able to take advantage of an anti scuffing additive and anti corrosion additive that are also contained in the 15W-50, but not in the regular W100. Of course this is an advantage to owners who don’t fly as often as they would like – a problem that I have myself – whereas frequent operations, such as flying schools, don’t need the additional performance of these additives and are fine using grades such as AeroShell Oil W100. This must work for many as AeroShell Oil W100 continues to be the world’s most popular aviation piston engine oil

So there you have it.

• Make sure your engine type is suitable for using a multigrade. All opposed engines from manufacturers such as Lycoming and Continental are fine; advice should be sought if you have a radial or inverted engine.
• Cold climates use a multigrade – but pick one with a low viscosity at start up – a AeroShell Oil W 15W-50 will out perform any 20W-50 in this regard. Remember up to 80% of engine wear happens immediately after start up whilst the engine is waiting for oil to reach the moving parts, so make sure you are doing what you can to get oil to your engine early.
• If you have a hot running engine – turbo or aerobatic – use a semi synthetic oil: again even though many multigrades are not semi-synthetic, AeroShell Oil W 15W-50 is.
• If you leave your engine for long periods additives can help protect the engine (AeroShell Oil W 15W-50 or AeroShell W 100 Plus), but take care when first starting your engine and pre-oil prior to start when you can.
• If in a stable, warm climate with temperatures consistently above 15 oC (60 oF), then oil circulation should not be too critical with single grade oils. Consider saving money and using a single grade oil. AeroShell Oil W 100 Plus can offer the protection of additives but without the expense of multigrades. Note that the ‘Plus’ in the title is what tells you it contains anti-scuffing and anti-corrosion additives.
• Remember to change oils every 4-6 months regardless of engine hours.

I hope that this helps to answer some of those multigrade questions that many of you have and helps you to decide whether it is right for you.

Happy flying.

Competitor’s Claims

I have been asked what I think of the testing used in advertising claims for another corrosion inhibited multigrade oil, and really I would advise people to take it all with a pinch of salt.

The corrosion testing that claims superior performance uses a test that is known to be very inaccurate – the variation in the results even according to the test method itself is 28% rising to a huge 141% error if the test were to be repeated by another operator. Remember this is the variation in results that can be expected when testing the same oil – hardly a robust position from which to be claiming superior performance. Independent testing has shown such a large variation in results that it is Shell’s position that it meaningless and misleading to use this type of test method for comparative purposes.

Again, we have all seen the wear testing advert showing some fatigue pitting on some cam followers. This is a non-standard test invented to show a positive marketing story. The test that runs continually (ignoring the effects of initial oil circulation) and uses totally unrepresentative conditions such as a 50% higher valve spring load that overloads the follower. This is nothing like what happens in a real engine and takes the parts beyond their design loads. Shell has commissioned independent testing using this technique and it has proven to be unreliable – the marketing claims are for results showing endurance up to 300 hours, whereas the independent testing showed failure of the cam after only 100 hours using the competitor’s oil.

So how does fatigue damage of cam and follower occur in real engines? Poor oil circulation causing limited lubrication and damage to the parts. How to prevent it? Use an oil of the correct viscosity grade and use an oil that circulates as rapidly as possible at low temperature. This test method produces nice photographs, but ignores the real issues.

The question with all of this testing is to ask how representative is it of real life and are claims robust if the differences shown are less than the accuracy of the test method?

Cam showing fatigue damage that started after only 100 hours

using Exxon’s own modified CEC L-30-T-81 test.

Oil used: Exxon Elite