(i know its old, but it still makes me laugh)

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

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 night before trackday. Synthetic oil stole
your neutral and sold it to the Chinese for $1.25. Synthetic oil
will make you grow a tail.