Can synthetic oil improve fuel economy? Yes. I DO agree with G-Man II, though. I cannot think of ANY circumstances that a synthetic oil could pay for itself based on fuel economy. IMO, at best, you can expect .5-1mpg (at the same weight/viscosity).
However, it probably could pay for itself with extended oil change intervals (OCI, drain). Rather than the typical ~3K mile drain of most "dino" oils, you probably would be safe with ~5K mile drains, even without doing an oil analysis, though I would highly recommend getting at least one on the synthetic oil after the engine break-in period.
As for the "sell the car before the engine wears out", you must mean prior to ~50-75+K miles, correct? I'm not familiar enough with the Chevy 4-cyl engines to say for sure, but 4-cyl engines typically do run hotter. Also, IMO, domestic engines, in general, are not manufactured to the tolerances that their foreign competitors typically are. They also tend to have greater variability in the engines. Because of this, they can wear more quickly due to abrasion and oil contamination. Both of these can lead to "sludging" of the oil and lower long-term reliability.
Anyway, these are just my thoughts on the matter. If nothing else, having records that you have used synthetics (IMO, Schaeffer's 7000 is a great value and oil) might make selling the S10 easier as the prospective owner might believe (correctly?) that you took good care of it. Won't help on a trade-in, though.
YMMV...
Scott