So I've just now noticed that the fire from Fire Jars won't light up oil on its own, although it used to. I'm not sure wheter it's a glitch or a change from people who complained about the oil slimes being overpowered when exposed to fire, but it's just silly when you think about it.
Only people/enemies that are set on fire can actually blaze the oil. Actual fire from fire pits, fire bombs and now the fire jar don't even affect it. I haven't tried it with the pyro alchemer gun, so I'm not sure...
Hopefully they do change that. Fire is meant to be destructive, right? That's the beauty of it.
I just tested it and what you said is somewhat true. I went into the training hall and threw an oil jar and then threw a fire jar on top of it. It didn't light it on fire, but when I ran into the fire it lit the oil on fire without lighting me on fire. I was able to walk away without catching fire. It's definitely something that's changed recently, so I think it's probably intentional.