S'funny. All the cans of engine oil that I ever poured into an engine were round and opened by punching triangle shaped holes into opposite sides of the lid with a key that had another end that was useful for opening bottles. Oddly enough, I noticed the chap topping up the hydraulics was using exactly the same key to open his cans of hydraulic fluid.
But we're civilians so what the **** do we know?
For your benefit then, ...the civil sector
increasingly won't use them as they take up more room during shipping.
I'm not going to argue this point, the bare facts are round tins take up more room than rectangular ones on a standard shipping pallet and hence cost more to move about as shipping costs in marine and the road freight industry is generally based on size of loads not weight. If you are still using round ones then you are paying more for the product than if you were using rectangular tins