A slight slant.. On an RV fridge they say 3 degrees. I once worked out how far out of level, side to side, with an 8' wheel base 5 inches over 8 feet is just shy of 3 degrees and thus acceptable.
On a residential fridge. The cooling system will work no matter the angle. However modern Residential units are "Frost Free" or so they falsely claim.
They still frost. but after the compressor shuts off a heater melts the frost and it drains down from the Freezer to the Fridge and then down into an evaporation pan under the fridge..
Off level by a MAJOR amount can prevent the drainage. espically if it tipped Towards you (Front down) and it can cause the evaporation pan to overflow onto the floor no matter the angle (The drains are at the rear of the Fridge) .
