The number one question I get about lubricants is “multilayer versus one grade, which is better?”
I have wrote several columns in the past about this, but I will try to shed more light on this topic.
Let’s start with the basics: viscosity is defined as “the internal friction of a fluid that causes it to resist flow”.
A specially designed test tube is used to measure the viscosity of the oil in the laboratory. The test oil is placed in a test tube and then immersed in a temperature-controlled bath. The tub has a side window, and when the test oil reaches the desired temperature, the oil is allowed to run out. The time required for oil to leak through two special marks then correlates back with viscosity.
In the past, viscosity was reported in seconds, but now it correlates to centist, an established unit used to measure kinematic viscosity. I guess it sounds more scientific.
The oil bath is controlled to 100 ° C (212 ° F) for the high temperature test, but for the low temperature test it is different for different masses.
A graph of viscosity at high and low temperatures is then plotted, and the slope of the line is shown as a viscosity index.
In the mineral oil business, there are usually oils with low, medium and high viscosity indices. All oils for reciprocating aircraft engines are blended with HVI base oil or with a high viscosity index.
There are basically two ways to mix colored oil. You can use a synthetic base oil with an ultra-high viscosity index or mix a polymer additive that acts as a viscosity index improver. You can also use a combination of the two.
These polymers have long chain molecules that shrink when cold and then expand when heated. The polymer does not slow the flow of oil at low temperatures, but expands by reducing the flow rate of oil at high temperatures.
If you mix two oils, one monochromatic and the other all-round with a polymer additive, so that both oils have the same viscosity at 100 ° C, will they both work the same in a fully warmed-up aircraft engine?
Not at all. You have to remember that the viscosity measurement was done by gravity, but your engine is running at a much higher oil pressure. This higher pressure can compress long polymer molecules and even cut them off forever, which can increase flow.
This is good news / bad news.
The big advantage of universal oil in cold start. During one test in the lab, we filled out the Aeroshell 15W-50 test engine crankcase. After a short acceleration we leave the engine overnight in a cold room with a temperature of 20 ° F. In the morning we turned on the engine and measured the time from the moment we started spinning, until the oil enters all parts of the engine. We then conducted the same test with Aeroshell W 100 solid oil.
The universal oil lasted for four seconds, while the 100 class took more than 10 seconds. So this is a definite advantage for a comprehensive oil.
On the other hand, all-purpose oils tend to leak more than single-grade oils. So if your engine has significant leaks, you can expect more leaks with universal oil.
This raises the debate about whether it will damage your engine’s transition from one grade to a versatile one.
I’ve never seen anything like it.
If your engine has oil leaks or you have leaking intake manifold guides, then switching from one grade of oil to a comprehensive one is likely to increase oil consumption.
But if you don’t have any significant leaks and the main source of oil consumption is past the piston rings, then you’ll probably see a 30-50% reduction in oil consumption when switching from a single-grade oil to a comprehensive one.
This is due to the improvement of oil flow in the area of the annular tape, which is due to the increased efficiency of the oil rings.
Improved universal oil fluidity characteristics also help reduce the fuel consumption of your engine.
During the annual test at a major flight school, part of the fleet used the Aeroshell 15W-50 and the other part used the monochrome Aeroshell W 100. The test showed that fuel consumption on aircraft with universal oil was about 4% less than part of the fleet using Aeroshell W 100.
These savings may help offset some of the higher prices for comprehensive oil.
But how will this affect the engine of my plane?
Earlier, I noted that the high pressure in the engine of your aircraft can compress long polymer molecules and even cut them off forever, which can lead to increased flow.
This makes some people worry that it can lead to increased wear over time.
I have found that enhanced flow with universal oil will compensate for the shift in most bearings that are hydrodynamically lubricated, such as crankshafts and cams / hoists.
The only exceptions I have noted are a slight increase in the wear of some large radial main rod bearings and sometimes exhaust valve tips, especially when the aircraft is operated in very high temperature conditions.
So what oil should I use on my plane?
So, what is my best advice on which oil to use?
It depends on where you live and the climate in which you fly.
If you live in the mid-US, then mix it with one variety in summer and multiple in winter.
If you live in a warm climate, you can use the oil of one variety all year round. However, if you are planning a trip north in the winter, add a universal oil change for this trip.
In fact, using a universal oil in the summer works well in most situations, but using a single grade oil in the coldest winter can cause problems with starting and even solidifying the oil cooler.
https://generalaviationnews.com/2022/02/14/multigrade-vs-single-grade-oil-which-is-best-for-your-airplane/ What is best for your plane? – General purpose aviation news