On Wed, 04 Jan 2006 02:05:57 GMT, "hondaman" <jeffscomp RemoveThis @hotmail.com>
wrote:
>I would just go with what the dealer recommends. that's usually the best
>thing. i'm wondering about that weight of oil myself because i have a real
>old civic that calls for 5w-30 and i wonder if it would be better with
>5w-20.
Here is what I learned about the chemistry of motor oil about 8 years
ago (I needed the information for a technical paper). The information
may be out of date, but I would think it still applies.
So called multiviscosity oil (a misnomer) contains hydrocarbon
polymers. Polymers are molecules built around long chains of carbon
atoms, maybe 100 or so atoms in the chain. Plastics are polymers. DNA
is a polymer. Polymer just means a long chain of similar units.
These long molecules are tightly coiled up at low temperature (due to
hydrogen bonds) but at higher temperatures, the molecules uncoil and
get tangled together. This "tangling" effectively gives the oil a
higher viscosity and is what causes the viscosity of the oil to remain
higher as the temperature is increasaed.
The problem is, as I understand it, that for oils with a wide
difference between low and high temperature viscosity (such as 10W-40)
such a large quantity of polymer must be added that the polymers
begins to gunk things up. So it is best to stay with an oil where the
two numbers are closer together.
My Honda owners manual recommends 5W-30, but allows 10W-30 in certain
temperature ranges. Down here in Texas (where the temperature right
now is 81 degrees) it never gets cold enough to neede the lower
viscosity oil, so I always insist on 10W-30. The polymer problem MAY
be why Honda and other manufacturers have gone to 5W-20 as a
recommendation.
On the other hand, there are many characteristics of motor oil that
are a lot more important than actual viscosity. As another poster has
said, these include such things as film strength, ability to hold
contaminants in suspension, etc. Modern motor oils are far superior
to the stuff I had to pour in my car 50 years ago and are an important
part of the reason why automobile engines can easily go 200,000 miles
or more with no problems.
Just my $0.02 worth.
Elliot Richmond
Freelance Science Writer and Editor
>> Stay informed about: 5-20 oil on 98 Accord-I4?