Phil, I've done extensive research and read a lot regarding this very issue.
Bottom line, there is no detrimental effect from switching from convention
to syn and then back again. However, if u do "top off" btwn oil changes,
yes, you should
use what is already in the engine, i.e. either conv. or syn. Do not
mismatch. All the name brands claim to make the best oil. I'm not a mechanic
but my research points to valvoline as the mechanics choice, whether u
prefer conv. or syn. (I have no affliation with valvoline nor do I profit
off advertising their oil)If u live in the north were it freezes a lot, syn
is a better choice because it's properties perform better than conv. at
lower temps. I live in the south where it rarely freezes, and several local
oil change places tell me to use 10W-30 vice 5W-30. Their thought is the
thicker oil is better in warmer climates. Not true. If u do the research,
the reason the OEM (orig equip manufacturer) recommends 5W-30 is because
today's engines are machined to a much more finite tolerances and therefore,
5W-30 (or thiner oil) is recommended. Again, lot's of good data to be found
once u filter out all those name brands and know it all's who think they
have the right answer. Don't take my word...do the research. :-)

I've been absent from newsgroups for a few weeks due to work, and just
saw this.  I hope the following comments will still be of value - at
least to someone, if not Phil.  Up front, I'll disclose that I have a
lifetime dealership with Amsoil (an opportunity I don't believe they
still offer that was only available for a brief period many years
ago).  HOWEVER: I do not actively sell Amsoil anymore.  I use it
solely for my own vehicles, and have no financial interest in the
following, since I am no longer an active dealer.  With that said, I
personally would not use any other products in my vehicles - I just am
too busy to keep up active selling/promoting, so I stopped that many
years ago.

The vehicle make isn't a problem.  We've got Amsoil products front to
back (wheel bearings, auto. transmission, engine, rear end, air
filter, oil filter, etc.) in our 2001 Rodeo.  But we made the change
after the first oil change (bought it brand new).  You mentioned
further down about adding oil between changes.  If that's the case, it
may significantly depend on where a quart is going every 3K miles.
I've never added oil to our Rodeo, and never added oil to a '96 Isuzu
Hombre a year ago that had around 120,000 miles on it (Amsoil from the
beginning, of course).  The problem with a switch after running
conventional oil for 80k and no idea how good the oil change cycle
was, is that you don't really know how much wear there is, OR how much
sludge, etc., has built up because of oil quality, infrequent change,
or just plain 80k miles on petroleum oil.  If you're using oil now,
it's quite possible you'll use MORE oil if you switch to a synthetic.
(See comments below about switch-over procedures.)

Absolutely!  At least with Amsoil (and presumably most of the others,
depending on type), synthetics can be an outstanding engine cleaner.
If you just dump in synthetic oil with no good (and careful) engine
flush first, you can make things worse.  It's not unheard of to have
some cracks in seals that are plugged with sludge, etc., but have
those seals start leaking after the cleaning takes place.  That's one
of the concerns I have about buying off-the-shelf synthetics - there's
no real education.  A good Amsoil dealer won't just sell the oil,
they'll educate and help make it the best possible experience.  I had
a customer many years ago who switched over at around 40,000 miles, as
I remember.  He would not use a flush, and wouldn't even use the old
recommendation of dropping a quart of old oil and replacing with a
quart of Amsoil, and driving for 500-1000 miles (I think it was).
Then make the change.  The quart of Amsoil would clean the engine
pretty well, but not as well as a good quality flush.  Things went
well for around 10,000 miles, then suddenly he started using oil
significantly.  Problem?  After oil analysis it turned out to be that
the additive package in the oil was completely depleted due to having
essentially been used up cleaning out everything else, then holding it
in suspension (if not in the filter).  We changed the oil again at
that point, and he never changed oil again to at least 70,000 miles
(and never added another quart of oil).  Yes, he used analysis from
that point on - he was sold.  But that points out how important it is
to clean the engine well prior to the switch.  I'd be concerned about
doing it with 80,000 miles of unknown history.

None at all.  It's going from conventional to synthetic that can be a
problem, ironically because the synthetic can be such a good cleaner
(at least Amsoil - I cannot say for sure how much that's also true for
others, as I don't have any experience with others).

If you do have to add oil, I fully concur with the other writer.  It
makes no sense to pay for the excellent protection of a good
synthetic, and then dilute it with petroleum oil that's going to
create more sludge, etc.  Plus, a good synthetic's viscosity won't
change as much as a petroleum, so why mix?  Either you want all the
benefits of synthetic, or you don't.  Don't spend the money if you're
not going to make it well worth your money with max protection.

As for viscosity, I've used 0W-30 ever since it came out.  Amsoil
gives it a change interval of 35,000 miles or one year, with 6-month
filter changes in between.  The other answer talked about using the
lower viscosity range (5W- as opposed to 10W- ) and is correct.
Amsoil just happens to have come out with a 0W-30 so I switched to
that.  By the way - it took care of some cold-weather engine noise
shortly after startup when we first got the vehicle going into winter.
It was reportedly due to inadequate lubrication in the valve guides,
but I don't know that for sure - I just know it hasn't returned.

The superior lubrication and consequently lower operating temperatures
extend the life of engines.  Plus, it just doesn't break down like a
petroleum oil will.  So for someone like me who tends to keep a
vehicle until it's just about ready for the junk yard, the cost
savings end up being enormous despite the higher initial price with
the synthetic I use.  I had to replace the timing chain in my Hombre
at just over 100,000 miles, and had my mechanic run a compression
check before deciding whether to make the investment in the new chain
or sell the vehicle.  Compression was almost at factory specs, and
there was less than 5 pounds difference between any of the cylinders.
I wasn't surprised - the mechanic was, though, particularly since
there had only been about 8 oil changes in 8 years (an never added oil
except at filter change time).

Again, I don't sell Amsoil, and haven't for many years.  I just have
been using it since 1978 and was long ago convinced of its superior
performance (research backs that - not just anecdotal tales).

C.R.