5th dnd – What is the DC of a Grease Effect put together by the Slipper Oil?

The slip oil states:

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

The Grease spell says:

Slippery 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 fat appears, each creature in its area must make a Dexterity save throw or be prone to falling. A creature that enters or completes the turn must also make a dexterity save roll or be prone to falling.

What is the DC rescue spell of a fat put together by a slipper oil?