Definitely urban myth, go to this website and reach your own conclusions.
http://www.bobistheoilguy.com/

As far as I know, a synthetic oil is better, more stable and more homogen
than a 'natural' oil. And it is more expensive to produce, therefore the
higher price. I do not believe in the urban myths about be incompatible with
mineral oil.
I use a 5W-40 on my XJ40 3.2 (AJ6-motor) from 1992, no problems what so ever
for the first 45.000 km I have done. My Citroën 2CV from 1975 is running on
this oil as well, and this little high-revving, aircooled motor does crave a
good oil.

Leo

I found this wonderful piece of humorous writing somewhere on the Internet.
Written by someone called Luca-4.

----------------

Beware of synthetic oil, it can do terrible things to you and your
beloved motorcycle. It will not only leak out of your engine faster
than you can put it in, but it will also cause your oil filter to
clog and implode, dumping debris and dirt into your lubrication
system. It also will make every part of your bike permanently
slippery because of its linear molecular chain dispersion action.
Then it will leak onto your kickstand causing it to retract
automatically, dropping your bike on the ground!

But that's not all...

Synthetic oil will round off your gears and spin your bearings. It
will also splatter onto your seat causing your girlfriend to fall
off in the apex of a turn and she'll never ride with you again.
Synthetic oil coats your sight window and your timing window
with a  whitish pro-emulsification additive that is both non-
removable and highly corrosive. Synthetic oil will completely
leak onto the ground overnight and your dog will drink it and die.

Synthetic oil will wear out your tires and make your battery leak.
It will give you the desperate need to urinate after you put your
full leathers on and then jam your zippers shut. It will contaminate
your gasoline causing your bike to stall on railroad tracks and
accelerate uncontrollably near police cars. It will make it rain
during rallies  and on weekends.

It will lubricate your timing belts causing them to  jump teeth and
break your valves to bits. Synthetic oil chemically  weakens
desmodromic valves and causes the clearances to change
every  six miles. Then it melts the black soles of your riding
boots right before you walk across your new carpeting.

While riding past groups of attractive women it will cause both of
your handlebar grips to slip off at the same time so you smash
your  windscreen with the bridge of your nose. It also causes your
swingarm to crack, your studs to break, and your rotors to warp,
and then it voids your warranty by changing your odometer
reading to 55,555. It  also dries out your wetclutch and wets your
dryclutch. It makes your clutch slave cylinder seal fail in the
heaviest traffic on the  hottest day of the year while putting an
angry wasp in your helmet for good measure.

Synthetic oil hides your 13mm socket and puts superglue on your
earplugs. Synthetic oil will scratch your faceshield and make your
gloves shrink two sizes right before trackday. Synthetic oil stole
your neutral and sold it to the Chinese for $1.25. Synthetic oil
will make you grow a tail. Synthetic oil will write long crazy
e-mails to your Internet friends and then sign your name at the
bottom!