hi robert - i gave up on trying to say something useful on this subject
after my last post, because most people are going to say that the person
who is saying something about it - one way or the other is biased.  

i figure that anybody who does the lawn mower test with their own unit
will definitely see a different between synthetic and mineral base -
regardless of brand.

i will shed some light on mobil one since there doesn't seem to be
giving credit where credit should be due.

first, mobil one was developed back in the early 60's and was discovered
by accident.  they brought in to the lab where they use to do a 392 hour
dynamic engine oil test.  the purpose of this test is to run the engine
for 392 hours solid, then the oil drained, and sent to the lab for
analysis.  the synthetic oil sat there behind the door for a couple of
years and when the lab boys ran out of oil to test, they ask about this
stuff that had been sitting there.  there was a assigned number on the
box.  well, they put the oil in and did the test.  when they went to
drain the oil out, it still looked as clean as when they put it in, so
they poured it back into the engine and ran the test again.  after the
second time, they did drain it and sent it to the lab with a note of
wanting to know what kind of oil it was.  and that's how it came to be.

there were a lot of problems with ester based oils.  the amount of type
of additives to put it to make it compatible with mineral oil based
oils.   people don't realize what all is put into engine oil.  for
example - seal sweller.  that's right.  it has to be a certain amount to
cause the seal to swell at the same given rate the parts are shrinking
from wear.  if you didn't, you was leak more and more oil as the engine
got more wear on it.  and that is just one of many.  anti foam agents is
another, sure don't want to pump oil around in an engine with air
bubbles in it.  after they achieved the final balance it hit the market.

people were highly skeptical of an oil that was advertised that it could
go 25,000 miles without changing.  and it was 5 dollars a quart back
then.  could change a lot of mineral based oil for 5 dollars a quart.  

and then everyone people wrote in about what they put the oil through.
one person owned an airline company and they would send off their oil
samples of their engines to find out when the oil was so contaminated
that it needed to be changed and also based on the metal makeup what
kind of engine trouble to expect.  so, they started to send in the
synthetic oil into the lab.  the only thing they changed with the oil
filters.  it was over 100k miles on the oil before the lab said change
it and with no sign of engine part failure.  the lab company was
independent and had no way of knowing what  this particular product was.

another person did basically the same thing and at the 100,000 mark, and
when they mike the parts, they were still within original factory specs.

in the late 1960's.  the daytona 500 made news.  because of the track
temp being around 180 to 200 degrees and the big block race engine was
stuck next to the heat of the exhaust of the car in front of it - that
after the race, when they drop the oil plug, no oil would come out.
they would drop the pan - take a pocket knife and literally cut the oil
out of the pan, rolled into a ball and could bounce it on the floor.
total oil breakdown.  the ones that had ran mobile one, not only did the
oil run out of the pan, but when they tore the engine down, they found
that it still have the cross hatch pattern in the cylinder walls.  these
stories were carried by popular science and popular mechanics at the
time.

look how long it took for the other brands to finally come out with
their own synthetics and of course, the name of the game is to knock
down the leader.  in this case - mobile one.  

but they are all the same base makeup, just as all the oil companies use
mineral based oil for their regular oil.

so, which one is best?  i'm not going to side up with one of the other.

but, i will tell you this much.  synthetic oil is not suited for extreme
high pressure, extreme demand applications, such as use in mining
operations.  when you have a 7,000 dollar pump the galls up because of
the type of oil being used, it gets expensive fast.  mineral based oil
clearly is the better choice.

but, for light pressure, high temps, hostile environment applications,
such as in autos and trucks, they appear to be the best choice, but it
depends again and the person's driving habits as to whether or not it
would be in their best interest to use them.

i hope this will shed some light on the subject.

~ curtis

knowledge is power - growing old is mandatory - growing wise is optional    
"Many more men die with prostate cancer that of it. Growing old is
invariably fatal. Prostate cancer is only sometimes so."

No one blows an engine (due to lack of lubrication) while using API
certified oil that has been changed regularly and kept up to level.
-------------------

i will address stan's comments.  

i have studied oils since 1961, and have taught  lubrication at
automotive and industrial businesses.  the catch phrase is changing oil
regularly.   there are so many variables in this dept.  

for example - on mineral based oil, the API classification is based on
the temp. of 180 degrees in order to come up with it's standards.  but
the society of automotive engineering states that for every 20 degree
rise in the base line, will cut the life of the oil by 50%.  so, if one
was to put this oil in the family station wagon,  pack it full of kids,
and people plus luggage, add a heavy trailer to drag down the road,
turn on the a/c, and head for the mountains on a hot summer day.  what
is the life of the oil?   could be over in less than 2,000 miles.  now,
if someone was into abusing the engine, then many of the properties of
the oil have changed and may not offer the protection as fresh oil.  

i could go into synthetic oils, but let me say this since it is coming
upon the growing season.  take your old trusty lawn mower out, chank it
up and made some passes and push the mower to the choking point, but
don't let it die.  notice where this point is.  then, if you want.
change the oil with fresh mineral based oil and do the same thing.
after you are satisfied where that choke point is, then put in synthetic
oil in.  (doesn't make any difference in brands) and go cut the same
lawn and notice where the choke point is.  remember, you haven't done
anything to the mower, just change oil.  and if you see any improvement
on a 3 1/2 hp engine, think what you would see if you have a 150hp
engine.   then write back to the newsgroup and tell what you found out.
and tell how slow the synthetic oil gets dark.  then you decide whether
or not it's worth the difference.  

it's not me telling.  people say put up or shut up.  well, there's the
challenge and it's your mower, your lawn so how can it be fixed?  

you will notice a difference, but you will be surprised of how much.

knowledge is power - growing old is mandatory - growing wise is optional

My '86 Lincoln Mark VII has 188k miles( many miles of towing) and 18
years with cheap oil, and my '92 Mercury Grand Marquis has 203K miles
with cheap oil and rarely needs additional oil between changes. I have
no doubt both engines will far outlast the usefulness of the
interior/exterior and other running gear of the vehicles. As long as oil
changes are done regularly (~5000 miles) and the oil level is kept up,
engines will outlast the rest of the car every time. Main cause for
engine lubrication failure is low/no oil or extending intervals so long
that some tiny oil passages get clogged. People don't check.
Stan K.