Actually I'd like to confirm to this by saying that it's not the Oil consumable itself:
But the actual amount of variaty goods in the inventory.
I had been using oil without issues, but then I added dates as my 9th consumableand suddenly my variaty moral boost dropped to 2 from 7.
Upon selling the dates it went back up. And I can actually swap any of the consumables around as I like, it's only when adding the 9th that it shows this behaviour.