Ok - I'm gonna jump into the middle of this - and admit I didn't see the
original question and possibly some followups - and risk the (perhaps
deserved) flame I'm gonna get - but here goes....

I have been a firm believer in synthetic oil since it first came out - yes -
way back then.  I normally change the oil in my cars (which obviously don't
tow trailers) every 20,000 miles.  (BTW, so far, I've stuck to Mobil 1).  I
just sold a Mazda 626 (1987) (surplus car is the only reason I sold it).  It
had 350,000 miles on it - the mechanics told me it had less internal noise
(wrist pin, etc) than some new engines he has heard.  Didn't use or leak a
drop.  Granted it was a Rice-burner engine.

On the trucks, I change my Isuzu Trooper every 20k also (can't tow a whole
bunch with 120 ponies).  Engine still like brand new at 100k.

I'm changing the oil in my F150 at 10k intervals since I tow a Hi-Lo with
it - I'd guess about 5,000 # on the hoof - of course without the wind drag.
The F150 has the 3.55 rear end so it has to pull and shift a lot - granted -
(BTW - see question below).  It has only 40k on it but so far, quiet and
solid.  Hope it stays that way.

I had an old Mercury Capri that I tore the engine apart at 215k to rebuilt
after running Mobil 1 in it the whole time.  (The only reason I went ahead
and rebuilt it was I had to take it out of the car cause I had the car
lowered (4" clear - handled nice though) and ripped the engine mount screws
out of the block on railroad tracks - yes it was a young and stupid time).
When I took it to the machine shop, they wouldn't believe it had those miles
on it and kept insisting I must mean 15k.  (this includes cam, cylinders,
etc).

Maybe my experience is unique but I'm sold.  I've had really good luck with
the synthetic and would recommend it to anyone - whether towing or not.  And
no, I'm not a distributer, salesman, etc, etc, etc.  Just sharing my
experience.

Now to my question:  I can't seem to find an answer on:  On towing with the
F150 (5.4L).  It does a lot of downshifting from 4-OD to 3rd on each grade
while towing.  Would you recommend disabling the overdrive to make it stay
in 3rd (with the inherent higher revs) or let it shift?  I get different
answers from every dealer, mechanic, friend, foe, etc.

Thanks,
Papa Don

I agree with you up to a point.  I also tow most of the time with my truck and
change oil every 5,000 miles.  The reason I change at 5,000 miles is, I have a
100,000 mile extended warranty from Ford, and it states that the warranty is
void if the oil is not changed every 5,000 miles and makes no mention of
towing, so I don't change at 3,000.  I usually get another truck at around
100,000 miles anyway, because by then, all the accessory parts are failing and
becoming hard to find.

Tom J

I've changed oil even more frequently than 3K miles when my wife was
commuting less than 2 miles to work each day.
The problem is that the engine never quite got up to actual
running/operating  temperature, which caused sludge to begin forming
from accumulation of water condensing in the crankcase oil.
It was also MURDER on my muffler, which dripped water long after the
engine was shut off.
Synthetic oil helped by not being as susceptible to this as conventional
oils.
My humble experience.
Summary: How often you change oil should be dictated by the type of
driving you do, how frequently you can afford it and your own personal
convictions on what works best for you/yours.
I choose to "over"-maintain my Broncos.
I figure the peace of mind and longevity are the payback....
Even using synthetic oil, I change/drain the crankcase at 3k or less.
Trans fluid (convertor drain too) and filter at 25K or less (also
synthetic). Transfercase (syn ATF) @ 10K or less.
YMMV

Randy