RE: Pep Boyz

[Scott Fisher <sefisher@cisco.com>]

Steve Buchholz says:

> If I were making the choice I would be more likely to
> vote for not having to replace the engine ... and be 
> oblivious to the $$$$ I've tossed in the $h!tter by my
> lower fuel economy ...

Old saying: "Oil is cheaper than metal."  Is gasoline?  Let's look.

I've read fuel economy savings reasonably quoted on the order of 1-2% by
going with lightweight oils.  A few moments with a calculator yields the
following results (which I'm going to express in miles per gallon and
dollars; I can waste enough time to do that, but you'll have to waste
your own time figuring it out in your local measurements and currency
:-).

Let's look at a car with a baseline average fuel economy of 20 mpg, over
a 100,000-mile interval.

100,000 miles therefore takes 5000 gallons of gas.

Raising the fuel economy by 1.5% (to 20.3 mpg) uses 4926.1 gallons of
gas to go 100k miles.

Rounding off that 0.1 gallon gives us a 704-gallon savings over 100,000
miles.

At the price of the gas that's in my tank today ($1.89/gal), that's a
savings of $1330.56.  Probably not enough to replace an engine, though
possibly more than the value of a 100k-mile Audi considering that these
cars shed resale value the way a white cat sheds on a black velvet
dress.

Now, I've ignored a LOT of variables (inflation in oil/gas prices over
the time represented by 100k miles, added cost of oil consumption due to
increased wear caused by lighter-weight oil, etc. etc. etc.)  And I
haven't factored in the consideration that the reduced drag from lighter
oil essentially gives you either another 1-2% of economy *or* another
1-2% of power, but not both at the same time, and we all know what
*we'd* do with the choice.)

Note that one of the initial incentives for using synthetic oil is that
the molecules were designed to provide both low friction AND high shear
strength/thermal stability, again on the order of a few percent -- not
magic, just thorough engineering.  You may reasonably expect both
improved economy and improved engine protection by going with a 10W-50
synthetic, but at what cost?

That same 100k mile service interval, assuming 3000 miles between oil
changes (we're all maniacs on this bus, right?), gives us 33.3 oil
changes; let's assume we made one longish trip and didn't change the oil
for a couple hundred miles and round it to 33 oil changes.  I'll also
use 4 quarts per change in these calculations, as that's how much new
Castrol I poured into the GT last Saturday.  And finally, I'll use an
average cost per quart (again, US figures) of $1.25 for dino and $3 for
synth (ignoring filter costs).  (Those prices may be way off; I'm
ashamed to say I don't remember what each quart of oil I bought last
Saturday set me back.)

This gives us, over that 100k-mile service interval, a cost of $167 or
so for oil changes with mineral oil.  The cost jumps to $396 for $3
synthoil, meaning that the person who changes his or her oil every 3000
miles with synthetic oil will probably still come out ahead over 100k
miles -- even assuming only a 1.5% increase in fuel economy and the
improved engine protection of a higher-viscosity oil.  So it sounds like
you're still about a thousand dollars ahead after 100k miles by using
$3/qt synthoil, because you save the gas and you don't have to replace
the engine.  (And yes, I know Audis can go a lot longer than 100k miles
between engine replacements; I just used these numbers because they're
easy to think about.)

--Scott