I recently received an email from Mark Peterson concerning the use of preservative oils. I have addressed this before, but preventing rust in low usage aircraft is still the number one lubricant area of concern for GA pilots.
A number of years ago, Phillips started to recommend that pilots add a quart or two of a Mil-C-6529C oil to an oil change just before putting an aircraft away for an extended period of storage. This product is available from Phillips as Aviation Anti-Rust oil and from Shell as Aeroshell Fluid 2F.
This preservative oil was developed during World War II to keep spare aircraft engines from rusting during the long sea voyage from the U.S. to Europe. It was also used in new and used aircraft that sat idle for extended periods.
The military would run the engine, usually at idle for about 15 minutes, and then shut it off and seal up the ports. They would also hang a bag of a vapor phase inhibitor (VPI) in the crankcase prior to shipping.
These oils are the only anti-rust products that I know of that are approved for all aircraft piston engines. They are approved for fly away types of operation, but a limit of 50 hours operation over the life of the engine is recommended. Much of the time limit is because it is in a straight mineral oil with no ashless dispersant to maintain engine cleanliness.
So what is the best way to use this product? It depends on a number of factors.
For example, if you live in a desert or dry area, it may not be needed unless you are storing your aircraft for an extended period. If you live in Seattle or similar humid area, it is more important.
It is also very dependent on how much the aircraft is flown.
If you fly about 50 to 100 hours a year and leave your plane sit for only three or four months, then I would suggest you change the oil prior to storage and add a quart or two of the preservative oil as part of the oil change.
With this mixture you can fly your plane whenever you like, but remember these oils contain a coating additive and you do lose some of the effectiveness of the additive when you fly on it.
It is always critical to change the oil every four months or if the hours guideline comes earlier.
If your plane will be sitting for much more than four months in a humid climate, I would suggest you consider putting in just the 6529C product. You can still do some flying on it, but now you should follow the general guideline of about 50 hours total.
This becomes less critical if you fly a lot of hours in the summer since the ashless dispersant will clean up your engine.
Pilots have also asked if you can reuse the preservative oil the following year.
You can, but I could find no data concerning how effective it would be. I know that it does lose some effectiveness.
There is not a good answer for the really low usage aircraft. I have talked to pilots and mechanics who service planes that fly only five to 10 hours a year.
With a 2,000 hour TBO, it would take 200 to 400 years to reach that and I doubt the preservative oils will guarantee that.
Many of these planes are in museums and are just flown in a few air shows in the summer.
I would recommend that these aircraft be changed to the preservative oil just before storage. And then an ashless dispersant oil is used during the short show season to clean up the crankcase somewhat and then back to the preservative oil.
In addition to the right oil, it is very critical that your oil temperature be in the 180°F range during cruise and that you always have clean oil prior to storage.
Also, do not go out to the airport and just run the engine for a few minutes just to hear it run.
If you start your engine, you hit the primer a few times to get it going. This puts raw fuel in the cylinders and some gets to the crankcase. The sulfur in the fuel combines with the moisture that is present to form acids.
Unless you get the oil temperature up high enough to boil off the water, you have done more harm than good.