An honest to goodness - non-corner cutting - Synthetic Oil is formed using synthesized chains rather than a petroleum base oil. Some companies have been known to use highly refined oils while calling them "Full Synthetic," which they can get away with in the US due to lower Regulations. The question is, what does that change? I don't know for sure...
Though, back to Conv vs Synth, on mile #2,500, your conventionally oiled engine will exhibit more wear in its warm up cycle than the 2,500 mile synthetic. It's incrementally negligible, but over time can be significant. The conventional oil will tend to stick to parts and creates more build up over time. The stuff that makes it into the intake will run less readily, and will create larger gunk deposits in your intake ports on the heads, thus disrupting airflow (again, incrementally).
So, it isn't BS. It's just... how long will you have the car, and what do you expect to get out of it?
Conventional oil is not going to damage a motor. There are new chemicals on the market that handle the gunk just fine (like sea foam). Most people won't own a car for long enough to see the difference. However, there IS a big difference, and it all has to do with flow after it starts to break down.
In a high performance motor I consider Synthetic to be a MUST. Most manufacturers are even moving to synthetic for their I-4s. Tigher tolerances require cleaner and smaller molecule chains.
What is Synthetic Oil? - Synthetic Oil vs. Conventional Oil