I don't have evidence at my fingertips, but I believe it's pretty well documented that synthetics do four things dino oil won't: last longer, better low temp performance (less friction), prolong seal life, and to a lesser extent reduce wear. I run synthetic oil in all my engines - on the little stuff like lawn mowers the cost is not a big deal, so why not; on the big stuff, it makes the engine perform better and last longer with less maintenance so it's worth it, and the best reason - it gives me longer between oil/filter changes - that saves me all kinds of time/hassle.
Buy from Walmart on sale if you can - cheapest I've found.
I think synthetics in drivetrain gearboxes is a good bet too, but I've not done that yet.