[Vwdiesel] Risks of straight weight
val at mongobird.com
Wed Oct 5 08:35:19 EDT 2005
A few comments...
> Do any of our oil chemists know what the dangers might be running non-diesel
> rated, ordinary 10W30 motor oil in say a 1.6 Jetta TD in cold Canadian
> winter weather 0 C (32F) to -25C maybe -30C ?
Biggest factor between CD and SH (just to use two oils) is
in soot handling capabilities. The CD is about 3X the SH.
Change your oil every 3000 or so, and it's not a big factor.
I have a 91 NA Jetta that has 200K miles and recently had the
head pulled (failed gasket secondary to failed thermostat
rivet). The engine has mostly been run on "cheap" oils.
Some Castrol, but mostly Mobil and Shell stuff that sold
at low prices. Straight weight in the summer, and 10W30 in
the winter. A few changes of Rotella 15W40, mostly the first
couple of winters.
Valve cover clean, head top clean. Compression prior to teardown
was 495 +/-5 PSI across all cylinders. The machine shop
servicing the head even commented on the cleanliness of the
The Mobil cheap grade oils were high in detergent, and that
helps wet the metal surfaces and reduce sludge accumulation.
James is right, and high levels of soot accumulation are not
a good thing. If you're having sludge accumulation, you need
to change more often, and if you're not using a CD oil, then
you should consider doing so. You should also ascertain why
you have the sludge buildup. Blowby, fueling, etc.
> What is different about a lube oil formulation for a diesel engine than for
> gasoline engines anyway?
Soot, scuff and buffering, last I checked. But the specs keep
changing. That was a CD to SH comparison. The specs are online.
I'm a scientist, but I needed a tribologist to help me understand
them. I'd encourage you to look at them.
> > A question for the experts: What percentage of the total volume of oil
> > would viscosity improvers have to be to make a a straight 15W oil look
> > (15w)40 oil. 1%??? 60%??? If it's only 1%, then there is still 99% of
> > lubrication available in the mix. If 60%, then I would guess the oil
> > only have 40% of the lubricating capacity of a straight 40W oil (ignoring
> > any lubrication aspects of the additives).
The numbers you are asking are situational, and depend upon the
manufacturer, and their current formulation. Thirty years ago
I bought cases of STP 10W50, thinking I had found the universal
multivis, at discount store prices. One of the triboligists at
work analyzed it. I can't find the report at the moment, but my
recollection was that the non-mineral oil components comprised
almost 50% of the volume. The wear testing was poor (expect any
current formulation to be much better), and essentially I was
convinced within a couple of hours to never use the oil in anything
I really cared about, at least for very long.
> > Far as changes are concerned, oil doesn't really "wear out", but the
> > additives do get chemically neutralized and the soot accumulates. IMHO,
The oil does change it's properties, but it is true that the
additives may deplete faster, and that oil contamination may
indicate changing. Using a "cheaper" oil, it's real easy to
just change it.
> > Other hand, synth oil does a better job handling a hot turbo on shutdown -
> > less prone to coking i.e. converting liquid oil into solid carbon (doesn't
A very good reason to use a synthetic on a turbo. Also a
good reason to consider how you operate the engine. As an
example, while it's not a factor on airplanes because they
have taxi time, on turbine helicopters we run the turbine
at ground idle for 60 seconds to stabalize the temps prior
to shutdown. Pulling off the turnpike into a rest area,
such a practice might be a good ideal on a blown VW.
> > > I've always run Cennex 15W/40 year round, even when it's -20F.
> > > That doesn't happen much anymore but other than slighlty slow
> > > cranking on the COLDEST mornigns it does fine. Pour point is
> > > the real thing on cold starts and it has a lower one than a lot
> > > of "thinner" oils.
> > > Loren
Pour point varies between different manufacturer formulations,
and will vary with use and debris in the oil. There are
specs for the oils. Again, I'll reference a noontime session
with the tribologists. They took a quart of oil, used in
corporate fleet equipment, and stirred in a pound of "fines"
which is another way of saying dirt and soil and sand.
At 70F, the oil held the pound of "stuff" and visibly flowed
OK, and I was told would wear test acceptably.
The oil was chilled to 0F, and it was like tar. I was told
that the same oil, at 0F would only hold about two table
spoons of dirt. Something like a 30:1 different going from
70F to 0F. So in the winter, when I know it's going to
get real cold (which here is -20F to -30F), I simply change
the oil. The new, clean stuff always seems to crank better,
and I've even used 5W30 stuff. Car starts like it's summer.
Could I be causing harm? Is it what Bentley or the owner's
manual says? Here's the way I look at it. Manufacturers
are under pressure to extend oil change intervals. It affects
cost of ownership figures on cars. Oil providers are under
pressure to support those extended intervals with new formulations.
Even in gassers, it may not be a good idea, if you're really
interested in maximum service life. While the conservative
approach of using the rated oil, in the rated weight, is
generally accepted, there is a knowledge base which says that
the decisions are not as clear cut. Many of us try to use
the best knowledge and techniques available to run our cars.
We tune injector timing differently than the Bentley procedure.
We look at the smoke, we listen to the engine sound. We can
blotter test oil, we can have our oil sampled and analyzed
(cost permitting), we can cut open oil filters and check for
metal, we can track compression, etc. Using S rated oil, and
changing frequently (3000 miles, perhaps more, perhaps less),
may be economically advantageous. Using Rotella 15W40 is
conservative, and will likely serve you well, as well.
Choices are a good thing.
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