Swaid,

Sorry about the name misspelling - now that I think of it I used to know someone named Swain....  One problem with these lists is that it is tempting to abbreviate posts to save time.  Sometimes a longer answer is better, and this is no exception - so I recommend that anyone not particularly interested in this topic hit the delete button now :-).  As for oil change intervals, there is no one factor that determines the "best" oil change interval, even if there is a single "best" interval.  A long time ago, far, far away (in Detroit) I watched with interest some testing done at GM.  It was shortly after the change to lead-free fuel and the question came up about oil change intervals, which I think at the time were recommended at 2,000 miles.  We had lots of company cars that were typically driven for 6 months to a year and then sold.  The "policy" was that we never serviced any of them.  Should we have?  So we took a rather large fleet (some competitor's, too) and drove them with various oil change intervals.  We found that there was very little difference between 2,000 mile and 50,000 mile intervals.  If you looked at the statistics, you might barely detect a slightly higher incidence of engine trouble with the higher intervals, but just barely.

Then I worked on a project with oil experts from an oil company.  Their opinion was that you could look at oil and with a little practice see that it was ready for a change.  Lead is gray, carbon is black and water is...well, sludgy.   Their point was that modern oil filters do a pretty good job and oil by itself doesn't "go bad", it just gets contaminated.  Are there invisible contaminants?  Maybe, but what are they and where did they come from?  Then there is the simple logic that there is no such thing as "good" and "bad" oil.  The oil gets progressively more contaminated, so it never goes over the cliff and suddenly becomes bad.  There was an excellent post that stated that the multi-viscosity additive (VI improver) broke down with use (actually, shear rate and temperature) and gradually became thinner.  True, but at the same time the lighter ends evaporate, so does the oil viscosity go up or down with use?  It can do either.

And there is the history of aircraft engines.  Oil change intervals have been recommended at 50 hours for many, many years.  I remember it being 25 hours with no filter and 50 hours with a filter.  What has happened since?  First, the oil itself has a much better additive package that reduces "aging."  Multiviscosity oils reduce the demand for changing oil on account of weather.  Synthetic oils have a much more stable viscosity.  Oil coolers have thermostats to keep oil warm in the winter.  Piston rings and cylinder materials and finishes are better than before.  Lead content, one of the bigger contaminants, hasn't changed much, but with LOP operation much less of it finds its way to the oil.  So in summary, many conditions have changed and almost all would indicate a longer oil change interval - but the recommendations haven't changed.  And why would the OEM's increase the recommendation? They have absolutely no incentive to do that, as it doesn't save them any money and can only cost litigation.

Then there is cost.  I'll admit I have never paid for an oil change, but I would guess it would cost about $200 most places.  Just the oil and filter cost over $50.  So if you change the oil and filter every 25 hours, at 2,000 hours you have paid about $16,000 for oil changes!  That's almost enough to overhaul the enigne.  Let's say you get aggressive and change the oil every 75 hours, 50% longer than "recommended."  At 2,000 hours you have saved $12,000 on oil changes compared to the 25-hour changes.  How much longer would the engine have lasted with frequent oil changes?  100 hours?  200?  Even 500?  You can see where this is going.  I have heard the comment "oil is cheaper than engines, so I'll change it more often."  Is it?  How much oil do you have to buy to increase the life of the engine and by how much?

And do the manufacturers know what they are talking about?  Sure, the engineers - and their lawyers - are smart guys.  What is published comes from the lawyers, not the engineers.  I still maintain that if an engine is "intelligently" operated it will run a long time with extended oil change intervals.  How do you run an engine with intelligence?  Don't shut it off until the oil has been "cooked" enough to drive out the water.  Don't store it for more than a couple of weeks.  Don't keep it warm in the winter.  Don't get the CHT's much over 400 and cooler is better.  That's about it.  Shock cooling - never mind, that's another topic.  So when do I change oil?  I target 50 hours (see, I'm a coward that is unwilling to follow his own advice!), but that's when I start thinking about it.  If I'm on a trip I certainly won't change it until I get home.  I'll probably wait until the oil is burned down a bit - no point in changing oil I just put in.  So the bottom line is that the part-synthetic oil will get changed never sooner than 50 hours and hardly ever later than 75.  And the oil is almost never changed at annual inspection time, as that never seems to coincide with an oil change interval.  False economy?  Don't know for sure - nobody does - but I've run multiple engines like that well over their TBO.  I'm satisfied.

I've already taken far too much of everyone's reading time and for that I apologize.  

Gary

Gary,

My name is        Swaid             not                Swain.           You have some interesting procedures for the care and feeding of your engine. What data do you have to back up extending your oil change intervals? Is it that you just don't want to spend the money?

Also all contaminates in oil are not black. There is also a calendar interval for oil change regardless of operating hours, it's to prevent corrosion inside the engine due to condensation.

 Also there is more to a compression test than just the number. There is a real neat place to get the right information on operating and maintenance of your engine. It's called the manufacturer. They have a vested interest in helping you get the best service from their product. Believe it or not they really know what they are talking about.

 Best of luck with your TBO,

Swaid Rahn