Full disclosure – the following is a silly question.
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.
There is already a precedent to making an item replicate the Grease spell, namely the Oil of Slipperiness:
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.
An Alchemy Jug is capable of producing up to 2 gallons of mayonnaise per day. I'm wondering how many gallons of mayonnaise (and thus how many Alchemy Jugs) would be required in order to replicate the effect of the Grease spell, if it can be done at all.
Best Answer
Up to your DM*
*(they mayo may not allow it)
Shockingly, the spell does not say what kind of condiment-based equivalency would be needed to achieve the same effect (or even if it even could). Thus, this kind of situation would be entirely up to your DM.
It is worth noting that there would be almost no way of even adjudicating this based on real-world knowledge. The spell effect is not related to the amount of oil on the ground, it is a spell effect. And even if it was related, there's no way to know how much mayo could possibly even try to replicate the effect.
In the end, it is unlikely to matter in any case except in the situation that this becomes a commonplace occurrence such that it is outcompeting actually casting the spell (or a related ability). In which case, being that it is negatively affecting the fun at the table, the DM should adjust accordingly.
As someone who has dealt with (as a DM) and caused (as a player) wacky mayo-based hijinks (mayo-hem?), the best method is just to go with whatever sounds fun at the time and keep it consistent unless a problem crops up.1
1 - Tip: If you don't want material-based shenanigans, don't give your player an alchemy jug. It is the first thing every player I have given one to has done with it and really the only way to make it applicable to most campaigns.