It would depend upon more info such as service severity, current lube choice, filters utilized, etc. If you want a specific OCI recommendation, you'll need to be much more specific in the details.
Generally, the temperature is moderate enough in HI that synthetics are not needed. Only if the OCI were to be greatly extended would syn's even be worth considering. If not, they would be a total waste most likely.
Being a delivery truck, does it idle a lot, or is it shut down at each stop?
A UOA at 5k miles is a good place to start.