Yes, the posters who warned about only supporting the car with a jack are correct.
Always use one floor jack stand to raise one corner, two if you support one side completely or the front or rear, and four jack stands if you want both ends of the car raised. Floor jacks operate using hydraulic fluid and the hydraulic cylinder can fail, dropping the car on you. The screw jack that came with your car will probably not mechanically fail but they are very prone to slipping if the car is moved or rocked at all (such as when tugging on a wrench).
How long your oil lasts depends upon the condition of the engine when the new oil is put into it and your driving conditions. High quality synthetic oil is comprised of a high quality base stock (ideal group IV synthesized) and a package of specially prepared additives. Nearly all of the additives are sacrificial (meaning they are designed to be used up) as they either allow the oil to do things the base oil by itself couldn't such as acting multiviscous, or they are designed to counteract or combat heat or contaminants.
Unfortunately, brand new oil introduced into an engine that has been poorly maintained goes under rapid changes in a matter of weeks as the additives sacrifice themselves to fight to counteract the contaminants and potentially sludge already adhered to the engine walls.
Further, warmup and slow moving operation in cold ambient temperatures (10C and under) typically causes the engine to run rich and dump fuel and soot into the oil in the form of cylinder blow-by. And if the engine never warms up completely (short trips) water vapor will condense into the oil, further contaminating it. When the additives are used up the oil changes for the bad. It typically thickens and runs hotter and if all of the dirt suspending detergent is used up contaminants will settle onto engine parts and be cooked (sludge), robbing the oil of one of its most important jobs - removing heat.
Oil ages by the hour of operation not the mile. A person who averages 60mph in driving habits will run the oil 200 hours in 12,000 miles of operation. A person who averages 10 mph in stop and go traffic in a major city will run the oil 1200 hours in 12,000 miles. For the stop and go situation not only are the hours six times as long, but the operating conditions (either slow warm up in cold ambient or hot operation in hot ambient) during those longer hours are much much harder on the oil
A rough rule of thumb is 250 hours of oil operation before changing oil. Is this chiseled in stone? Absolutely not. Conditions may determine lowering it to 150 hours or raising it to 350 hours, including the present condition of the engine. Additives in fresh oil will last much longer in a well maintained engine than a poorly maintained one.
But who measures hours on their car? Not many. Is the difference in cost in how you change your oil important to you? Calculations will show that if you change your own oil every 6K miles as opposed to every 15K miles, over 90K miles averaging 12K miles per year your extra cost is less than $6 per month of operation. Is peace of mind worth an extra $6 per month? I say it is cheap insurance.