I know it's hard to seperate the b.s. from the facts, but I'm always going to recommend a synthetic fluid over a conventional one.
I've worked in the trucking industry for many years. A person I worked with did a test in some transfer cases for 6x6 ReadyMix Concrete Trucks. 4 trucks invovled in the test, 2 with conventional lube, 2 with synthetic. After 2 years the transfercases were pulled apart for regular inspection and rebuild.
The 2 with conventional oil need bearings.
The 2 with synthetic looked like new.
In particular, it was also found that a shaft was not getting enough lubrication with the conventional fluid (manufacturing defect in the t-case design), yet the synthetic fluid was managing to lubricate this part in the other two units just fine.
These same types of experiences have been found in gasoline engines.
The down side to gasoline or diesel engine use of synthetics is you can't get the payback.
With synthetic in transfercases, transmissions, and axles in heavy trucks can extend drain intervals dramatically. Eaton transmissions with synthetic are rated for never needing a drain interval
But with engines you get combustion gasses and soot into the oil. A filter helps, but it can't remove it all. Eventually the concentration of these combustion byproducts will impact the health of your engine. Thus, while you should be able to extend 3k drain intervals out to 5 or 6 k, going beyond that is not recommended. And if you don't extend the drain intervals significantly, the cost is much higher for synthetic.
So if you are going to make sure you change your oil at 3k, then you really can use any oil.
Yet I prefer the little extra that synthetic gives me, and I'll pay the extra for it.