Over the years I’ve found that any change a pilot notes when switching from their usual single grade engine oil to a multigrade oil is usually blamed on the new oil.
This includes — but is not limited to — early cam failures, ruined rings, overheating, and increased rust.
Usually, there may not be a real technical justification to explain the change, but more of a perceived change.
But recently an engine rebuilder called with a question I had not heard before. He asked if multigrade oils would increase the possibility of valve sticking in piston aircraft engines.
My first response was “why would you think that?” Then I started to think through the mechanics of the problem.
Intake valve sticking can usually be related to oil flow down the intake valve stem. The oil flow plus the elevated intake valve stem temperature can result in coking and the formation of hard carbonaceous deposits on the stem and guide area. This, in turn, can lead to valve sticking.
So, the thinking is, the more flow down the stem the more possibility for intake valve sticking.
And one of the big features of multigrade oils is they leak better than single grade oils.
This is where the story gets muddy because of the improved flow characteristics of multigrade oils. The oil ring scrappers tend to work more efficiently at scrapping the oil film off the cylinder walls. This usually results in a 30% to 50% reduction in oil consumption past the rings when a multigrade oil is used.
However, the improved oil flow characteristics of multigrade oil will usually increase the oil consumed through leaks. The oil passing through the intake guides acts like a leak, so more oil consumption.
This is especially true for Lycoming engines with the valves below the rocker shafts, which means more oil in the guide area.
The effect of flow characteristics can cloud the oil consumption debate, with many pilots reporting increased oil consumption when switching to a multigrade, while others report a decrease or no change. Even a report of no change in oil consumption can mean that the decrease in consumption past the rings offsets the increase past the guides.
The increased flow past the guides is compounded by the increased amount of the viscosity improver (VI) additive used in multigrade oils. These complex molecules can be broken down under high temperatures and form deposits.
Before you run out and switch to a single grade oil, you need to know this is not a major problem for everyone. It usually only affects engines running at high temperatures.
Thousands and thousands of aircraft have been run on multigrade oils with no problems.
However, this is just one of the reasons I usually recommend multigrade oils in the winter months and then switching back to single grade oils during the hot summer months.
I wonder if, in the future, all of the problems with aircraft piston engines will be blamed on having to switch to unleaded fuels?
“Why would you say that indeed!”
The root causes for valve sticking vary widely by make and model. Intake valves do stick, but exhaust valves are far and away a greater problem for air-cooled engines. Of those varied reasons, I can cite the following in no particular order:
Make and model
New running clearances
In-service running clearances
High crankcase pressure
Contaminated oil
High oil temperatures
Poor lubrication
Piece/part fabrication and material
Assembly techniques
High/low cylinder head temperatures
Mixture mismanagement
Valve train geometry
Maintenance practices
Each of these causal factors can be expanded upon, but with only some exception, choice of oil brand and grade is not one of them.
Continental valves stick for reasons that differ widely from Lycoming. Small bore engines stick valves for reasons unrelated to high-power, big bore engines. One size does not fit all. What’s more, valves don’t need to stick to be problematic. A valve that runs up and down can be slowed in its guide by clearance impairment due to any number of composite factors. This slowing compromises the smooth flow of air through the engine and accelerates valve train wear and tear. A stuck valve is an immediate problem. A slow valve is an immediate problem in the making.
Removing TEL from the fuel will have an impact on heat transfer between valve, guide, and cylinder head, but it’s just another piece in a very big puzzle.
Ben, what is an oil ring “scrapper”?
There are typically 3 rings on a piston. the top 2 are compression rings, containing the burning fuel of combustion.
The bottom ring is the oil control ring. Oil will be splashed onto the cylinder, so, as the piston move down on the power stroke, [ and the intake stroke], the oil ring is designed to ‘scrape’ the oil off the cylinder and pass it to the interior of the piston through holes in the ring groove. This reduces the amount of oil that is burned.
I concur that, “one of the big features of multigrade oils is they leak better than single grade oil.” Years ago, I first noticed this when I’d tip my small bread pan used for catching oil when I removed an oil filter. After pouring it into a container, I’d tip it up on end overnight. In the AM, I’d find a bread pan darn near dry when draining multi-grade oil. With 100 wt oil, there was always a film remaining. I also noted slight seepage of the more slippery multi-grade oils in my airplane engines … even a new one.
Since I operated the airplane in Florida, corrosion was an issue so I stopped using multi-grade oil. Now, I’m a snowbird and leave the airplane hangared in Wisconsin for the winter so I still worry about corrosion due to inactivity. I wrote about this in the article on engine corrosion a few weeks ago. The main point here is that I stopped using multi-grade oil altogether for several reasons. I have a nifty setup to warm the engine prior to operation IF ambient temps dictate and consider that inconvenience a small price to pay. Unless you’re flying in Alaska and the engine sits outside, I see little advantage using multi-grade oil in an airplane. Now then, in my vehicles … that’s a different story altogether.
I’ve had 3 stuck exhaust valves on cylinders with 350 hours on them, and did the ‘rope trick’ and reamed the guides. The values and stems were clean, as were the spark plugs.
The lead salts deposited on the spring base and the inner part of the guide are from lead in the oil.
Mike Bush, Savvy Aviator, has a excellent video on valve sticking;
https://www.youtube.com/watch?v=7ohsVvYbAaQ
With a recent cylinder change , our oil use dropped from 2-3 hrs per qt., to 8-9 hours.
Oil analyses showed a huge increase in lead in the oil, 3,000 ppm. A subsequent oil change at 40 hrs, vs 50 hrs , still showed 2,000 ppm lead.
I’m now going to do a 25 hr oils change to see if the lead gets back to ‘normal’ levels.
Also, I haven’t noted any increase in oil leaks when we changed from w100 to Phillips20w50, and Camguard, on our Continental GO-300. It leaks very little oil !
But, more importantly, the oil pressure comes up within a few seconds vs a LOT longer with the W100, whether it’s Summer temps or colder.
[ The oil is just carrying the Lead salts that deposit on the exhaust guides. ]
I concur with what JS stated. I don’t recall ever finding an intake valve sticking. I have seen many exhaust valves sticking. And it is no question that the buildup of lead and carbon is the culprit. I also concur with the mix of ethanol free auto fuel and 100LL aviation fuel. I’d use a gallon of avgas to 4 gallons of auto fuel. Of course I am referring to low compression engines originally designed for 80 octane or less. Did anyone mention Marvel Mystery Oil?
Marvel Mystery, according to a Shell rep who used to attend the IA seminars, is a lightweight oil with some coloring. His recommendation was, f you want to lower the viscosity by using Marvel Mystery, that you merely change to a 40 weight oil from a 50 weight.
I concur. I see very little benefit in Marvel Mystery in aviation use.
As for putting it in fuel, he said that you are reducing the BTU value of a gallon of fuel by diluting it with Marvel Mystery. He said to me “When do you need the maximum power level from your engine?” I replied …. “At takeoff”. He responded “Then why reduce the potential engine power by diluting fuel with a lightweight oil?”
Oddly enough, in 45 years of aircraft maintenance, I’ve never seen an intake valve stick. It is almost always exhaust valves that stick, and it is almost universally due to the build up of lead salts combined with coked oil burned onto the valve stem to form a carbon lead build up causing the valve to hang open. You want to eliminate sticky valves, stop burning 100LL fuel.
My personal preference is about 20% 100LL burned with 80% alcohol free mogas. I used that up until the local fuel distributor started adding an injector cleaning additive to the alcohol free mogas that damaged my fuel system, specifically, composite fuel tanks.
My decades of years and thousands of vehicles dealing with this problem….despite the constant excuses, it’s the valve train materials used, not the oil. Changing valve and guide materials eliminates the problem.
I look forward to the day when GA engines and airframes don’t require the care and feeding of a Rolex watch. This discussion reminds me of early development of the automobile, e.g., Model T Ford. If GA aircraft had evolved to the extent that current generation cars have, we wouldn’t be having these discussions about oils and leaks, collapsing wheel assemblies on hard landings, exploding vacuum pumps, ad nauseum.
Merry Christmas everyone, and SAFE FLYING! Do your pre-flights carefully! Regards/J