Many years ago I was a driver for our unit mess hall.
We used to carry hot coffee, soup and boiling water to heat C ration cans (I told you it was may years ago) in Mermites.
What is the temp of boiling water?
The interior of Mermites is aluminum and I think they are stamped. I don't think boiling oil would harm them even if they were welded.
My guess is that the insulation between the outside and inside is some type of expandable foam.
They also came equipped with black plastic gaskets that must be used to keep them from leaking and to keep the product being transported hot or cold.
Rumor has it that they could be used as a field expedeint beer cooler by putting the beverage in and filling them with ice.
I'm sure GI's would never missuse govenment property to that end.
It may also depend on how old they are. I'm not sure when the Army quit using them or if they are still being used by some guard or reserve units.
You might try it in one as an experiment. Worse case is you lose a couple of gallons of oil and if you put it in a containment vessel you could recover the oil and do an easy clean up.
Remember this too. When liquid cools in a sealed container it contracts. If the container isn't vented you may get a locked condition.
We once had a tanker trailer collapse because the driver did not stop and vent it periodically as instructed when he loaded hot liquid in it. It literally looked like a collapsed soda can and the landing gear was dragging the ground.
Hope this helps.