a few thoughts on engine oil

Before we wade into what can be an emotionally charged discussion with many closely held beliefs about which engine oil is best suited for radial engines, I would like to make a couple of disclaimers:  1.)  I am not a petroleum engineer.  2.)  I am not an engine designer.  Having said that though, here at Radial Engines, Ltd. we have overhauled 1000 or so radial engines, and although our knowledge base is not broad (we can barely tell the difference between an O-200 and a VW engine), we do see a lot of the same thing with radial engines.  Rather than draw a lot of final and absolute conclusions, I would like to share some of the observations that we have made relating to radial engines and the oils that lubricates them.

In the beginning there was mineral oil.  All of the radial engines found in Stearmans, Wacos, and Cessna 195s were broken in and initially run on mineral oil.  This mineral oil had none of the modern wear additives, no detergents or dispersants, and therefore after a few hundred hours left gray sludge deposits all over the inside of the engine.  Though these deposits certainly added to wear within bearings and other moving parts, the deposits also helped to preserve the engine like a dirty form of comsmoline.  I have disassembled engines that had newspapers from 1945 stuffed into the exhaust ports, yet were not corroded or rusty inside simply because the mineral oil sludge had protected the internal steel parts.

With no slick and slippery anti-wear additives, mineral oil worked (and works!) well as break-in oil.  During the break-in of a new engine there are two criteria that must be met with a compromise of opposing needs:  1. The master rod and bronze bushings must be slowly burnished in to avoid galling or seizing.  2.  The rings must be seated into the cylinder walls.  Number one needs a slow and cool run; number two a fast and hot one.  So, we start off the test run at 600 rpm (good for the bearings, not so good for the cylinders) and wind up a few hours later at 2200 rpm (hopefully the bearings are burnished by now, but really good for the cylinders).  Here at REL we recommend staying with mineral oil for the first 25-35 hours, or until the oil consumption has stabilized (indicating that the rings have seated).  Higher than normal power settings also help with seating in the rings.  I recognize that this is not the only philosophy today for engine break in, but it has worked for us and I am reluctant to change something that works.

So if mineral oil is such great stuff, why should we ever use anything else?  There are a couple of possible reasons.  Mineral oil does not hold up well to heat. At temperatures above 200 degrees F it begins to coke and stops lubricating.  That is the primary reason that the Jacobs oil temperature gauge (and most other 1930s vintage radials) is redlined at 200 degrees.  Many of the later opposed engines are redlined at 230-240 degrees.  It was not so much an engine design issue as that the later opposed engines had oils available to them that would stand up to much more heat and continue lubricating.

Cold 50 or 60-weight mineral oil is also really sluggish.  In the winter it is not uncommon to experience thirty seconds or more of zero oil pressure during a start up.  That equates to thirty seconds per start up of excessive internal engine wear.  A few folks have opted for engine pre-oilers, though there is the penalty of additional expense and weight with a pre-oiler system.  If you operate your engine in cold weather or in a cold climate, multi viscosity oil and an engine preheater are good ideas.

Oils other than mineral oil also have additives blended to stabilize, provide for less engine wear, suspend particles, and a host of other tasks.  Contrary to popular belief, there are no detergents added to modern aviation oil.  Detergents were metallic based, often accelerated wear, and are no longer used.  Ashless Dispersant (AD) oil is the modern alternative to detergents.  AD oil suspends the offending carbon, lead, or metal particles in the oil to be collected by the oil filter (assuming that you have a filter).  Without an oil filter the AD oil keeps the particles suspended so they can circulate around and around in the engine and accelerate wear (not a desirable outcome).

Which oil, then, is best for my airplane?  Ask ten people that question and you will probably get at least five different answers.  All the modern oils are manufactured to military specifications and are quality products.  However, not every oil will perform best in every situation.  I recently had a customer call to tell me about his oil experiences.  It seems that using mineral oil with 400 or so hours on his engine, the engine would dribble oil out the exhaust after shut down.  A friend suggested that he switch to Phillips 25W60 and give it a try.  He was calling me to tell me that after using 25W60 the engine dried up completely with no more dribbles.  Within a week a second customer called to tell me his oil story. He had used mineral oil since overhaul (500 hours earlier), but had recently switched to Phillips 25W60 to see what would happen.  Oil suddenly started leaking from every gasket & seal on the engine!  After two oil changes with the 25W60 he could stand it no longer and went back to mineral oil.  Within a few hours the oil leaks stopped and he is happy again.  I thanked both of them for their diametrically opposed and absolutely contradictory information.

So which oil is best?  Ask me and I will tell you that I am like the politician who was asked how he felt about a certain piece of legislation.  He said, “Some of my friends are for it.  Some of my friends are agin’ it.  And I am for my friends.”  In the end the real answer may be, “The best oil is the oil that is working well with your engine in the environment that you are flying”.