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?