What is the DC of a Grease effect put in place by the Oil of Slipperiness?

Question

The Oil of Slipperiness states:

Alternatively, the oil can be poured on the ground as an action, where it covers a 10-foot square, duplicating the effect of the grease spell in that area for 8 hours.

The Grease spell states:

Slick grease covers the ground in a 10-foot square centered on a point within range and turns it into difficult terrain for the duration. When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.

What is the spell save DC of a Grease effect put in place by the Oil of Slipperiness?

Answer 1

DC 13, probably

Another means of producing grease would be by casting it from a scroll. Spell scrolls generally have their DC determined by their spell level, and grease is a 1st-level spell, which means a DC of 13.

Magical items in general have their DC determined by their rarity, and DC 13 is the most common value used for uncommon items.

Answer 2

Neat question. Here's what the PHB "Spellcasting" section says on saving throws:

Many spells specify that a target can make a saving throw to avoid some or all of a spell's effects. The spell specifies the ability that the target uses for the save and what happens on a success or failure.

The DC to resist one of your spells equals 8 + your spellcasting ability modifier + your proficiency bonus + any special modifiers.

So if you don't have a spellcasting ability modifier and nothing else says otherwise, the DC for something like this should just be 8 plus your proficiency bonus. Because of the way the last quoted sentence above is phrased, the fourth term in that sum may be absent and ignored, so it stands to reason that the second term (if absent) would also be ignored.

If you do have a spellcasting ability modifier, then you should use that as though you were actually casting the Grease spell, since having creatures make a Dexterity save is part of the effect of the spell you're being instructed to duplicate.