Oil Change Time - Synthetic or Conventional

[4CEFEDV]

The last time I got the oil changed, it was done at the dealership and it says 15W40 BULK (CJ-4) - I'm just trying to figure out if they used conventional or synthetic oil so that I know what I need to buy this time around so that I can do the oil change.

2017 2500 6.7L I6 130,000 mi