My father worked HVAC for Trane for 35 years and had Chevy vans the whole time. They got whatever bulk 5W-30 the shop on the corner had in their tanks that day, and each van was "retired" (given to the low guy on the totem pole) with over 200k miles. This was from 1970 - 2006. There was one Ford in there, but he doesn't like to talk about it.
In reality, any conventional oil you buy now with the SN spec will be partially synthetic to meet the spec. You really can't go wrong with the cheapest one that meets API SN and a decent filter for 5k miles. At first glance, your usage of the van may seem severe but it's really not that bad. 5k miles may even be a bit short, but you'd need a used oil analysis to determine that.