Engine Oil

An essential part of preflighting any aircraft with piston engines, is to check the engine oil.  This is a lot different than your car.  I don’t know about you, but I sure don’t check the oil in my car’s engine every time before I use it!  The reason for this is that aircraft engines are built considerably “looser” than car engines.  Car engines are liquid-cooled and as a result operate in a fairly narrow temperature range.  Airplane engines are almost always air-cooled (to save the weight and cost and complexity of a radiator) and as a result can see a much wider range of temperature during operation.  Because different kinds of metals are used in engines, they expand at very different rates, requiring that an aircraft engine have much larger clearances, which allows much more oil into the combustion chamber.

This is why aviation oil is special – “ashless dispersant”, it will say on it.  This oil burns without leaving carbon deposits.  A long-dead friend of mine decided to save money and ran car oil in his Lycoming.  It mostly worked.  When it was rebuilt, the engine shop knew it was running car oil, because of all the pockmarks burned in the top of the pistons.  Oops.

So you can’t save money and run car oil in your Lycoming or Continental – sorry.

As a pilot, you’re going to be pulling the dipstick and looking at the oil level an awful lot.  But what are you really looking for?  I used to instruct on 172’s that were full at 8 quarts.  But if you actually filled them up that high, they would toss out oil rapidly until it settled down at 6 quarts.  This is because the oil in the wet sump below the crankshaft is whipped up as the crankshaft rotates, and if there’s too much oil, it goes out the crankcase breather tube onto the belly in short order, making a terrible mess.

I call this the “windage level” of an engine.  It’s handy to know it.  One trick I learned, is to add half quarts.  Let’s say your oil gets down to 5.5 quarts.  Just add half a quart, to take it back up to the windage level.  Or, wait until it gets down to 5 quarts, and add a whole quart.  Whichever you are comfortable with.

When it comes to oil, how low is too low?  Good question.  As always, the POH is governing, but often it will be mute on the subject.  And even the engine manufacturer’s operating manual and instructions aren’t always very helpful.

One thing that is good to know, is that Lycoming says that you only need a couple of quarts in the O-320 in the 172, to avoid metal-to-metal contact.  The rest of the engine oil is for cooling.  You need more oil in the summer, basically.

Also, the FAA says that to be airworthy, the oil sump must be at least half full for takeoff.  In the example of the 172 above which is full at 8 quarts, I would hope you would add a couple of quarts if you found it at 4 during the preflight!

When it comes to topping up the engine oil, don’t stress out if the only engine oil is a different brand.  For example, feel free to toss a quart of Philips 20W50 in with Aeroshell 15w50.  It will mix just fine.

One note I should make about engine oil level … if the engine is equipped with an air/oil separator, you can actually run it higher than the windage level without losing any oil.  For example, I strongly recommend aerobatic aircraft run full oil, all the time.  Their Christen 801 inverted oil system includes a generous “slobber pot” for this.  You will actually be slightly down on horsepower due to crankcase windage, but hey, that’s life with a wet sump.

As I mentioned above – and you probably already know – there are many different brands and viscosities of aircraft engine oil available.

The first engine oils available, ‘way back when, were “straight grade”.  For example W120 was thick (SAE 60W), W100 (SAE 50W) is a bit thinner but a good summer oil, W80 (SAE 40W) is a bit thinner yet, and W65 (SAE 32W) was what you ran in the winter. 

Frankly, straight grade oils are excellent in warm weather but they suck moose cocks in the winter.  People use to perform “oil dilution” with gasoline before they shut the engine down, and sometimes even drained the oil out of the engine and took it inside overnight.  Seriously.

In the winter, sane people use multi-viscosity oil, like Aeroshell 15w50 which flows MUCH better (and thus reduces metal-to-metal wear) than the equivalent W100 straight grade, which it has the viscosity of when it warms up due to the magic of plastic VI’s (Viscosity Improvers) which uncoil as they heat up.

There is another very good reason to use 15w50 instead of W100 in the winter.  It’s called the “pour point” of the oil, which is the temperature at which it congeals. W100 congeals at only -20C which isn’t very cold.  When that happens, the oil cooler plugs up and that’s not good.  It can also go solid in the hub of a constant speed prop.  15w50 has a pour point of -40C which is much, much better.

Some people like multi-viscosity oil so much they run it year-round, and if you’re rich, you can.  In the summer, I prefer to run Aeroshell 100WPlus which is a W100 (SAE 50W) straight grade oil which has had the Lycoming extreme pressure additive required by Airworthiness Directive for the horrible O-320H2AD engine lifters.  It’s an inexpensive (and very good) engine oil, for the summer.

I should talk a little bit about oil additives.  I am not a great fan of oil additives.  I figure if it was such a good thing to put in oil, Shell (or someone else) would have already done it.  There are two common FAA approved engine oil additives that I know of.

The original aircraft engine oil additive was Lenkite or Avblend.  Some people deride it as “Snake Oil”, and I will agree that the manufacturer of it is very obfuscating.  What Avblend is, is the most expensive (and FAA approved) ether you will ever purchase.  Ether is a solvent which dissolves carbon.  If your engine has a lot of carbon deposits – specifically on the exhaust valve stems – Avblend can do wonders in getting rid of stick valves.  For a little while, until the carbon builds up again.  If you have sticking valves, sooner or later you’re going to learn about Lycoming SI1425A which is known as the “rope trick”, where you mechanically remove the carbon from the exhaust valve stems and the lead from the exhaust valve guides.  Note that SI 1425A does not exactly tell the whole story about how to perform this, but that’s getting a bit outside the scope of this simple memo about engine oil.  I will mention that not only exhaust valves but also intake valves can stick and cause rough-running engines.  And bend pushrods.

A new engine oil additive is called “Camguard”.  Legend has it that it was developed for a new aircraft engine oil, but was determined to be too expensive.  So, they sell it separately by the pint which is good for about 10 quarts of oil.  I wasn’t a believer in Camguard until I saw the results of the corrosion in the humidity cabinet.  Google it for yourself.  For an engine (especially a Lycoming) which is privately-owned and doesn’t fly much, or an engine in a humid, salty environment, consider Camguard.  Lycomings have such a terrible problem with cam lobe and flat tappet lifter corrosion, they went to roller lifters, which is yet another subject.


acboyd@gmail.com   Jan 2012