2002 Mercury  are there any advantages or -disadvantages to using synthetic oil instead of conventional oil    it uses 5W-20?

The car's engine is already pretty well worn so using synthetic oil will not do much for you.   You might see a very slight improvement in gas but not enough to compensate for the increased cost of the oil.   I would just use regular oil and change it and the filter every 5000 miles and call it good. 

what the history of the car , has it always had sythetic oil or is it a high preformance car used for racing , basically its a waste of money , after all a good 50 oil/50 syth. semi would be the best for you , but its nearly 20 years old , a good 10/40 aws oil would do you fine .

YOUU BET YA"!!!! Wax based oils are just JUNK compared to real FULL SYNTHETIC OILS! THese items give off little to none acidic vapors to eat up rubber gaskets and seals or create sludge tra wax or VANISH within engine moving pArts! tHEY CAN WITHSTAND infinitely MORE HEAT WITHOUT DESOMPOSING INTO COMPONENET PARTS! tHEY LAST UP TO five times longer NAD KEEP ENGINE PARTS cleaner OR EVEN CLEAN AS SHOWROOM! senosrs REMAIN CLEANER LONGER AND exhausts AND PRICEY converters OAND o2 SENOSRS LAST LONGER, TOO! now>>>THERE ARE EVEN spec numbers TO DEAL WITH AS WELL AS viscosity RATINGS! Audi, E.G. REQURIES OIL WITH spec numbers 502/505 POM THE label! BMW REQURIES long life =-01,-03 OIL! BENZ requires spec numbers 229.01, 229.03 oil! These are FULL LOW SAPS oils created in EUROPE to match the LOW sulfur field we NOW have in this country! synthetics NEVER HARDEN coke INTO hot MOVING ENGINE PARTS, OR turbochargers EITHER. the NEWER "0"0 w SERIES OILS ARE EVEN BETTER, AND PROTECT AT STARTUPS AT 35 BELOW ZERO! this IS WHERE MOST ENGIEN WEAR OCCURS, AT start-up TIME! 

 can't say ENOUGH about the benefits of synthetic oils! The even can REMOVE the baked in tar and sludge reg oils have left behind! NMy FAVORITE is PENNZOIL ULTRA PLATINUM< that gives an amazing 500,000 mile EXTRA warranty on the engine with proof of purchase. I even =got ONE new engine from the manufacturer this way! Gm requires DEXZOS or better oils, and since most GM engines are vulnerable to TIMING CHAIN FAILURES< you are best off using PENNZOIL anyway.  VVALVOLILNE has put many engines directly into the JUNK YARD< cuz its made from ASH and TAR and returns to WAX and TAR! PENNZOIL is not made from crude oil but from natural gas. IT CLEANS as you drive and even removes many times years of bake din SLUDGE deposits made by inferior REGULAR oils! MODERN ENGIENS nearly all REQUIRE gull synthetic oil to keep the timing parts, and lifters and cams from collecting coke and tar. RUBBER HOSES and SEALS and GASKETS last longest and the pricey CONVERTER and O@ sensors are protected LONGEST. The list of PLUOSES goes on and on! LONGEVITY between changes AMSOIL is good for 2,000 miles with ONE filter change in the middle! MODERN FUEL injection systems are SO well controlled that less carbon is made to dirty th evil and SYNTHETIC Oil holds the carbon in a beautiful SUSPENSION till it is drained, not leaving TRACES of tar and deposits behind! But NOW all cars have SPEC NUBMERS which can be confusing! A synthetic made for GM< may NOT work in a JAPANESE car or EUROEPAN car that has a different chemistry going on!.. MODERN FUEL is now LOW in sulfur anew so the oil must match these characteristics. GOOD LUCK!   

Advantages: 5W-20 Mobil1 pours at a lower temperature than regular oil. This will allow  your car to start easier in cold weather. Mobil1 has stronger additives which will keep the inside of the motor cleaner and free of *sludge, particularly if you do a lot of city driving. Highway driving will keep moisture out of the engine better than city driving.

Disadvantages: Mobil1 is more expensive but higher mileage oil changes will make it well worth the extra cost.

You will LIGHTEN THE LOAD as you spend more paper money on Sin than conventional oil.

Advantage goes to the oil companies and the store that sells the oil. YOU LOSE.  Try reading the owners manual about what oil does the car NEED?  Good chance it is conventional. That is what the factor did all their tests with.

. What happens if say the Sin oil companies come up with an ultra Gold Sin oil (yes , it uses flecks of gold) Like $100/quart.  Are you going to buy that schit? Then you are dum. They are relying on advertising LIES to sell it. They make a hell of a profit and longevity of engine is no different than using conventional oil(REALITY)

 How is it gold?  They have a gold bar and the oil is poured over it and any loose gold atoms come off an thus into the oil.  In microscopic theory this may be the case. Reality is it is conventional motor oil. with a different label.

. A Mercury is a FORD (Found On Road Dead) or (Fixed Or Repaired Daily)

The synthetic oil can be used twice as long as conventional oil, as long as the engine is in good condition.

Synthetic oil breaks down less and lasts longer. 

There is a huge amount of ignorance surrounding fully synthetic oil as evidenced in these answers. Fully synthetic does everything better than conventional oil. There are no disadvantages and it mixes with conventional oil without a problem too. So if you want to extend the life of your engine use fully synthetic.

oil is oil ffs. some lasts longer than others before "breaking down"

I've never heard one being better than the other. All I've gathered is don't mix them or switch to one or the other being that the molecular structure is different and will cause leaks.

Damn. Do what you want with your vehicles. Good luck.