[s-cars] oil consumption

[Paul Gailus]

I've just heard anecdotal arguments that oils with higher viscosities
at ambient temperatures may have a corrosion advantage for
engines that sit unused for an extended period. For example, this
is mentioned at:
http://www.avweb.com/news/airman/184502-1.html

The oil's base stock for the most part determines the viscosity at
ambient temperatures, because the viscosity index improvers used to
increase the 100 deg C viscosity spec in multigrade oils haven't kicked in
yet.

According to
http://tinyurl.com/3vafz
or
http://www.olis.oecd.org/olis/2004doc.nsf/43bb6130e5e86e5fc12569fa005d004c/1
a84f68fcaee2076c1256f58004aff9a/$FILE/JT00174617.DOC
corrosion inhibitors are used because:
"Mineral oil alone allows permeation by oxygen as well as water
 and is thus not completely effective in preventing corrosion."
But I think this implies that maintaining an oil film is also important.
All I know is that oils with higher W (winter) grade numbers seem
to be noticeably harder to wipe off of a surface at room temperature.

The paper at the link below has a lot of quantitative info about
the effects of viscosity on oil film thickness and friction losses:
www.eng.auburn.edu/users/jacksr7/SAE2002013355.pdf

They say that with lower viscosity lubricants "there is a tradeoff
between reduced friction (and greater power available at the wheels)
and engine durability."

-Paul


----- Original Message -----
From: Trevor Frank <tfrank at symyx.com>
To: Paul Gailus <gailus at mindspring.com>; <s-car-list at audifans.com>
Sent: Wednesday, February 09, 2005 5:53 PM
Subject: RE: [s-cars] oil consumption



Huh is that true, I always assumed that the anti corrosion additives
where attracted to metal and therefore the relative viscosity or
"thickness" was not a driver.  I also wonder is the difference between a
25weight to a zero weight at what 180deg F is really enough to make much
of a difference.  I would think that the relative quality of the oil
made more difference.  Anyone have data on this?  I am very curious if
it would be true at cylinder wall temps...like 300C I think, at least
that is around the under the compression ring temp from what I think I
remember.

I have read that the big driver for lower visc oils is that the
protection is better with newer oils and that they want to reduce motor
drag..increase efficiency and reduce emissions.  That you can engineer a
motor for these lower visc oils using coatings and tighter tolerances
where necessary but to put a zero visc oil in a motor not designed for
it wouldn't be such a good idea.  Also zero visc is sort of a misnomer,
because the scale for an engine oil is at zero the centistokes or
centipoises is still a positive number...like anything it's all
relative.

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of Paul Gailus
Sent: Tuesday, February 08, 2005 9:30 PM
To: s-car-list at audifans.com
Subject: Re: [s-cars] oil consumption

I'm sure that there are a lot of factors affecting oil consumption,
but a higher viscosity will tend to leave thicker oil films behind on
the cylinder walls, therefore causing more consumption.
And zero oil consumption is not necessarily a good thing, because
it could imply that very little oil is getting to the first compression
ring.

I've also heard that oils with relatively low viscosity at room
temperature could make an engine more susceptible to corrosion
if it hasn't been started for a week or more. Not surprisingly, this
is because a thinner oil will eventually drain off and not stick to
parts like a thicker oil will. This has been observed with aircraft
piston engines, where some claim that the old straight grades are the
best for engines that aren't run everyday. So the 0W-xx grades may
not be the optimum choice for a car you don't run very often, especially
during the warmer season.

Paul


----- Original Message -----
From: <stefan13 at att.net>
To: <s-car-list at audifans.com>
Sent: Tuesday, February 08, 2005 11:16 AM
Subject: [s-cars] oil consumption


> During the recent timing belt install, my mechanic changed the oil and
put
Pentosin 5w-40 in the car.   I previously used only Mobil1 10w-40 (green
cap), which the car consumed at a rate of about 1 qt per 3,500miles.
with
the 5w-40, the car uses absolutely no oil, nada, zilch.  Somehow, this
seems
counterinuitive, and I am wondering if this is due to the viscosity or a
manufacturing difference.
>
> As always, thanks for any input.
>
> Stefan
> _______________________________________________
> S-CAR-List mailing list
> S-CAR-List at audifans.com
> http://www.audifans.com/mailman/listinfo/s-car-list

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