Dungeons and Dragons

Anytime my player use their actions and resources to interact with the world as of it's real I like to support it. I in turn treat the world real back which is what the player wants to do. We like to talk it through to make sure it feels fair to both of us.

For this example where your player is pouring oil and lighting a weapon rack on fire. So I would tell them that the oil will become visible once poured. So I would do a stealth check to get it lite. And either 1d4 rounds until the notice or give the enemy a check each round. Its fun to have these back and forth

An attack makes an attack roll.

I would probably not rule this as an attack. Lighting a creature on fire? Sure. But lighting oil, which happens to catch a creature on fire? Nah. IMO, aggressive actions aren't attacks unless they make contact with or directly (not indirectly) affect an enemy. At least, that's how I'd rule.

That being said, keep in mind that invisible creatures aren't undetectable , just unseen. Someone dumps out a flask of oil? As soon as that oil leaves the flask (so it's not being worn or carried), it's visible, and leaving a trail for any enemy to follow. Attacks against targets you can't see are made with disadvantage, but can still be made. A bunch of goblins swinging axes at the air are eventually gonna hit something. Are you having invisible characters make stealth checks? They'd get advantage, but if they make noise (e.g., strike a tinderbox), every enemy in the area should get a chance to roll a Perception check against Stealth, not just use passive Perception.

All that being said, if your players come up with a cool idea, roll with it. I actually really like the idea of an invisible PC lighting a fire on their enemies. But (most) enemies aren't dumb, and they're not going to sit around doing nothing if a clumsy, noisy invisible thing dumps out oil all around them and then lights it on fire. Plus, sometimes a half-executed idea is more fun than a perfectly-executed one (oh shit, remember when we lit the thieves den on fire but then had to run away because they were so furious and nearly killed Gary?!?)

Don't be afraid to change or define the rules as necessary. It's your game. If every single time they infiltrate, they're turning invisible and setting fire, then say, "it's going to be an attack from now on, and you'll lose invisibility." FORCE them to be creative. Lastly, let the players know that they can always ask. A PC casting invisibility would know if an action is going to cancel it. So they can check with you in the moment, you make a ruling, and then they can decide what to do. If later you decide you were wrong, tell them that next time it'll be different.

Hope that helps!

Similarly inexperienced opinion here, but I'd also allow it. Agree with the others that pouring the oil would be potentially visible, but definitely after the fire is lit the smoke would make the players visible when they move. Kind of like how the smoke allows you to see the tripwire lasers.

Rule of cool first and creative thinking is what makes ttrpgs great. However, you're the DM and the tone of the game is ultimately up to you.

Sounds like your players are thinking outside the box, which in my experience is nothing but a recipe for fun (and likely headaches for your carefully laid plans). Good luck.

I would allow it. The damage is terrible compared to actual attacks, the enemies can figure it out, and it's slow.

Let them have the win until it's underpowered in a few levels. It's fun to figure out a cool combo, and you can just make the game harder.

Strictly speaking, this is an item interaction, not an attack action. Clearly they're using it as an attack, and framing it as an item interaction to avoid losing invisibility.

I don't know the rules of invisibility off the top of my head, but I might do something like require a stealth check to maintain the benefits, or a perception or dex check from the other guy to notice it/avoid it hitting him. I don't think I'd actually end the entire spell, that has always seemed excessive to me.

I'm going by 2014 rules, but what constitutes an attack is actually pretty strict. Basically, there must be an attack roll of some kind, or the rules for that action must specifically describe it as an attack, for it to actually be an attack.

https://rpg.stackexchange.com/questions/71245/what-counts-as-an-attack

Personally I would allow pouring out oil to not break invis, but depending on the type of enemy, they might get perception checks to see if they can correlate oil pouring to mean someone must be pouring it for the players location. Although if they would throw the oil at something it would be like making a throwing attack with an improvised weapon.

Using the tinderbox I'd rule as attacking the oil in order to cause fire.

I feel like those moves equate to item interaction for pouring the oil right where they are, but deliberately lighting something on fire with a tool requires both more focus, intent and deliberate action.