The other advantage of synthetic oil-based grease is that it has about four times more film strength than mineral oil-based greases.

If your cup and cone bearings undergo bearing drop, perhaps due to a momentary overload, the balls have to slide sideways as well as roll. That's one instance where a higher film strength would seem to pay dividends.

With perfectly adjusted bearings, run at moderate loads, any grease should be fine. When things go out of adjustment, perhaps because they're new, or maybe the axles flex due to an overload, then synthetic pays off.