Question to Paul McBride, the General Aviation News engines expert from Ben Visser, the General Aviation News aviation fuels and lubricants expert: I have had several questions about oil change intervals since I wrote the column, “How often should you change your airplane’s oil?“
One guy said that way back in the past, the oil change interval was a recommended 50 hours or six months. If so, when did it go from six months down to every four months? I believe the answer is that the four months was a result of decreased aircraft usage.
Ben Visser
Answer: Ben, I thought you might get some response to the oil change intervals article.
I guess that the guy who said way back in the past the oil change interval was a recommended 50 hours or six months must be a lot older than I am, because to my knowledge, it’s always been four months or 25 or 50 hours depending whether or not the engine was fitted with a full flow filter or not.
I’m not certain that the four-month recommendation came about because of a decrease in aircraft usage, but that recommendation has been around for more than 50 years, even during general aviation’s heydays of the 1960s and 1970s.
I honestly think a lot of people believe the recommended oil change intervals is something the oil companies push in order to sell more oil.
They don’t seem to understand that it’s not the lubricity factor of the oil, but rather the contaminates in the oil that is the concern.
It’s obvious that they’ve never seen the inside of an engine that has had infrequent oil changes and the damage caused by corrosion.
I’d hate to think what some of those engines must look like internally that are still being flown these days that have sat idle for long periods of time with no oil changes and then the owner just starts the plane up and goes flying.
As you probably know, I continue to beat the drum regarding extended periods of inactivity and infrequent oil and filter changes, but there are still many non-believers who someday will pay the price.
Aircraft piston engine crankcases are designed with vents to equalize pressure during altitude/pressure changes… so they have a natural ‘air-pump’ action… even as air pressure changes occur on the ground/static air is moving in-and-out. Humid air entrapped in the crankcase is absorbed into the mineral oils where it combines with combustion/wear particle to form corrosive compounds. Running the engines with mineral oils to normal operating temps ‘bakes-out’ most of the moisture and resets the corrosion potential. Storing engines for long-term should include draining-old/dirty oil and then refilling with fresh/uncontaminated engine-oil [run-thru the engine for a short time]…. or refilling with ‘preservative’ oil for very long-term storage.
New synthetic oils have anti-corrosion additives and compounding that does NOT allow the moisture to be entrapped within the oil and combine with the combustion/wear debris… it precipitates out.
My dad always made a point of running all of his aircraft engines monthly… to heat/circulate the oils… which also ‘refreshes’ the rubber/elastomer/plastic/paper gaskets and seals which tend to ‘dry-out’.
NOTE. During engine runs, this is a normal/natural time to run/operate EVERY aircraft system… mechanical, electrical and electronic. The battery is refreshed/recharged… motors move, bearings are cycled, etc. Also, if practical… taxi [or simply tow-around] the aircraft: this allows wheels/brakes/shock-struts, etc to cycle and rotate and operate… and allows the tires to change roll to new ‘rubber-set-spots’.
One size fits all standards always gravitate toward the most conservative. Consider two identical aircraft, both flown limited hours on similar flight profiles. One is kept on an exposed tiedown in a high humidity location, the other hangared in a warm desert environment. A four-month oil change that might be appropriate for the first would be overkill for the second.
I formed a club & flight training with four aircraft (2 150M’s, 172L and a 177B 12 years ago and immediately moved each plane to Aeroshell 15W-50 semi synthetic. Kept all on a strict 50 hour routine with filter changes. The 172L we got with 350 hours and we will soon do a top end on the O-320 with 2100 hours on the cylinders (good compressions but, decided a top end was good sense. The others are all showing exceptional oil tests and compressions as they reach well beyond “normal” time to do the top end.
In automotive I have found that time doesn’t matter. That’s based on hundreds of engines having set for a decade or more and restarted with the same oil.
Results may vary.
This topic holds special interest to me given my past experience with aircraft engine overhaul. I’ve seen for myself what frequent oil changes can mean to the life of an internal combustion engine.
It has always been my belief that changing your oil every three months was related to seasonal temperature changes and a method to drain accumulated moisture from the oil sump. This is probably a throwback to pre-multigrade days when engine tolerances and machining was not as sophisticated as it is today. But, as a practical matter, it is the only way to remove acids from the oil. It matters little that oil companies want to sell oil; the acid needs to be removed, and frequently.
But if that’s not enough to sell you on the notion of frequent oil changes, understand that wear rates for piece parts that rely on oil for lubrication and cooling in air-cooled reciprocating engines is cut by 50% or more for engines in which oil has been changed every 25 hours and three months. This is a measured 50% in an overhaul facility, not wishful thinking, and gear teeth can tell the real story.
Indeed, inactivity makes everything worse but oil changes at 3/25 will go a long way in keeping your engine healthy.