We have a wonderful cantrip available since Elemental Evil: create bonfire. It's great for an example on how to handle PCs lighting fires to hurt people.
Disclaimer: Applying physics to D&D is often a bad idea, the rules work a certain way because of game balancing! Also, this is obviously houserule.
That being said, most long chain saturated hydrocarbons that would be commonly called "grease" will burn well once heated up enough. If you want to experiment with this, grab a candle and try and light the side. Nothing happens. Take my word for the other half of the experiment, that once you heat up wax in a frying pan or in a tin can on a fire, it burns vigorously. The same thing with deep fryers. That oil burns well once it's heated, but when cold, it won't light.
Based on this, my ruling has been that it takes a number of rounds to light up, and needs a wick (lit torch) to burn from unless a good fire based spell has been used to light it. Once lit, it does the same damage as a Create Bonfire cantrip, and if a creature passes through or begins their turn in the middle of the blaze, they will take damage (can't do a dexterity saving throw against something you're deliberately moving through). One whole D8 per round of damage. If they fail the save. Once it's had time to warm up and burn.
This interpretation is great for providing light, visual barriers, dangerous and damaging terrain, etc. However it limits its usage as a damaging spell. It also means it's not useful for putting out fires, unlike what was suggested on Twitter in response to Crawford's tweet.