Hi all.

I know this is going to sound extremely silly but I plan on taking a 5l sealed unopened bottle of cooking oil in checked baggage.
This is because cooking oil is bloody expensive at the destination that I am heading to.

Is this allowed in checked baggage?

5lt? Dunno, I have taken 1 and 2lt bottles of fancy Olive Oil without an issue.

There are usually warnings when checking in that you cannot take flammable substances. Is cooking oil flammable?

The FAA site (https://www.faa.gov/hazmat/packsafe/) indicates cooking oil is acceptable in checked baggage - see the chart found at the link.

Thank you all.

I've taken it (oddball material, not available at the destination), nobody cared.

You've gotten the answer to it's permitted from others, but my first reaction to your situation is this:

Five liters is a lot of oil. I'd recommend that you distribute it into smaller containers (all plastic, to absorb impacts better without breaking or cracking), securely tape the cap onto each one, and seal each one very securely with plastic bags and lots of tape. This way, any leaks will be contained, and even if a bottle breaks, you won't necessarily lose the entire 5L of oil. I would also pad each bottle well with bubble wrap, and make sure they're all in the center of your bag, surrounded by clothing and other soft items, to prevent impact damage.

I speak from horrible, first-hand experience. When I was in 6th grade, each student was required to bring in some cooking ingredients - I honestly forget why, whether it was for charity or for some cooking activity for the class - and I was tasked with bringing cooking oil. I put the bottle in my bookbag... and it broke open when I arrived in class that morning. I must have set my bag down too hard. The bottle was only about one liter, but that oil was like some supernatural creature. It soaked through the bag, it coated and soaked into every book, paper, and pencil, it leaked into my desk and onto the floor, and it was a nightmare to clean up. The entire classroom, all of the contents of my bag, and the bag itself stunk of cooking oil. I had to get a new bag, and all of my books were slick and smelled of cooking oil for the rest of the school year. I didn't throw the ruined bag away immediately, as being a 12-year old I was not the best at thinking ahead, so I tossed the bag into my closet when I got home that night. And thus my closet and even my bedroom stunk of cooking oil until I finally wrapped the ruined bookbag in plastic, and disposed of it in the trash.

So, to avoid what I went through as a kid, and to spare yourself the possible expense of needing to buy cooking oil at your destination, I recommend extreme precautions to keep the oil sealed up tight and away from everything else inside your bag.

Quick update.
I wrapped the 5l bottle of cooking oil in tea towels and then put it inside a bin bag and wrapped it up with duct tape.
It arrived in one piece at my destination to my delight.