Re: [A/S] Oil change monitors

["Carol and Oliver Filippi" <ofilippi@xxxxxxxxxxxxx>]

Sam,

Thank you for your reference to the GM web site discussing the modus
operandi of their computerized Oil Change Monitor system.

I was completely wrong (that an optical sensor measures contamination via
light transmission of the oil).  The GM system uses a computer model to
calculate the anticipated condition of the oil based on several operating
environment factors.

Thus we have an indirect calculation designed to predict the quality of the
oil.  Apparently this is a pretty good guide to the condition of the oil
given "relatively normal" operating conditions which track well to the GM
oil deterioration model.

On the other hand, it does not actually measure the properties of the oil.
Which is OK if the computer model is fairly close, and the determined
"change oil" threshold  is set to a conservative value.

My personal experience with manufacturer recommended oil change intervals is
that they are overly optimistic for my taste.

For example, I have found a noticeable increase in RPM (and quieter
operation) following an oil change at 3,000 miles (recommended oil change
interval 5,000 - 6,000 miles).  You would have a hard  time convincing me
that running the oil another 3,000 miles  would not cause more wear than my
changing the oil at the 3,000 mile mark.

Granted, I have done no chemical or physical analysis of the oil, but
obviously there is an increase in friction (and all the negative
consequences of it) by extending the oil change interval beyond (in my case)
3,000 miles.

I am very fussy about using a high quality oil in my vehicles.

Twenty or so years ago, a petrochemical engineer at Exxon's Baytown refinery
in Texas told a colleague of mine to "put the cheapest gas and the best oil
you can find in your engine."

With today's fuel injection systems one might want to modify his gasoline
recommendation, but I think the basic idea is pretty sound.  After all, the
oil is the only thing that prevents the moving parts from making direct
contact with each other and wearing. The additives in the oil control the
harmful byproducts of combustion which end up in the oil.

Since oil is cheaper than metal, I would prefer to err on the side of too
frequent an oil change interval rather than too infrequent.

The other side of the coin is that the oils (particularly the additive
packages, friction modifiers, detergents, etc. and the viscosity improvers)
have improved significantly over the years.  Nowadays, the engine
manufacturers build their products to tighter tolerances which reduces blow
by (and oil contamination) and also permits the use of lower viscosity oil
(thinner) which reduces friction and heat.  In addition, oil filters have
improved as well. In addition, with todays improved roads we put a lot more
miles on a vehicle per month or year than we used to.

I am not a proponent of the old 1,000 mile oil change interval, but I do
think some of the current recommended oil change intervals are a bit of a
stretch.  So far a synthetic oils are concerned, they should hold up longer,
but I have very limited experience to make any valuable comment on them
except to say that they take the heat a lot better and are slipperier than
mineral oils.

Oliver Filippi