Early synthetics got a bad reputation for leaking. This was because, despite the claims of the oil manufacturers, the seal-swell characteristics of the new synthetics were different from those of the mineral oils they replaced. If the seal-swell rate was lower, the seals shrank and oil leaked from crankshaft seals and rocker cover seals. If the rate was higher, the seals swelled a little extra and the engine was tight. Then if the owner changed back to mineral oil, or added a quart when no synthetic was to be had, things got really bad. The crank seals had become worn, in their turgid state, and then relaxed. The valve cover seals were compressed when swelled, and when the different oil was added, everything leaked like, well, an old English sports car.
Fortunately, the situation has improved; you should have no problem switching back and forth. Adding a quart of mineral oil to a crankcase full of synthetic will be fine. Read the fine print — a lot of the "synthetics" on the market are blends containing a substantial proportion of mineral oil.
Are synthetics worth the extra cost? Universally, the answer is yes, whether for a high-revving turbo motor or an older engine that gets little use. Using another vivid comparison, French champagne is also worth the extra expense over sparkling wine, but it comes down to matters of your taste and bank account. Note, too, that most auto manufacturers do not specify synthetics, so unless you're one of the few who need synthetic oil, rest assured that you're doing no harm with good old-fashioned crude as long as you follow the owner's manual recommendations on viscosity and grade.
On the other hand, synthetics are better on a number of levels. They keep the engine cleaner through improved sludge and varnish protection, reduce engine wear at high temperatures with more stable viscosity, protect the engine when it's running under severe conditions at high temperatures, provide better cold-temperature starts with faster oil flow at ignition and improve fuel efficiency.
One urban myth surrounding synthetic oil is its compatibility with conventional oil. At one time, those who switched from conventional to synthetics had to stick with syntheticsthere was no going back. Synthetics expanded the seals in the engine; then, when conventional motor oil was used, that engine sprang multiple leaks.
Now, while the companies don't recommend mixing or switching back and forth, one company's synthetics are fully compatible with others and compatible with conventional oils. The source of compatibility problems was high levels of ester in the earlier synthetics. Considering the consequences, it's best to verify this compatibility, either on the company's website or with your trusted mechanic.