Weil McLain is the largest manufacturer of cast iron boilers in the USA, and I expect the largest manufacturer in all of North America.

Here is Weil McLain's "Products" web page that shows all of the different types of boilers currently available from Weil McLain.

Residential Boilers, Commercial Boilers | Weil-McLain

Note the "2" at the top of the product selection indicating that the first page only shows the first 12 different kinds of boilers. Weil McLain offers 9 different kinds of residential gas fired boilers, 4 different kinds of oil fired residential boilers and 7 different kinds of commercial boilers.

On the second page it calls three of their commercial boilers "gas oil" boilers, which to me means that they can be set up to fire on either kind of fuel; and could undoubtedly be switched from one to the other relatively quickly and easily. I could see a demand for that feature in places like Alaska where gas may be temporarily interrupted by an earthquake that severs the gas lines. Places like hospitals and government offices could have their own oil storage tanks and switch over to burning oil until the gas service is restored.

What I don't see, however, are residential boilers that can burn either kind of fuel or a conversion kit to allow a residential boiler firing one kind of fuel to burn another. What I do see, however, is that Weil McLain's GV90+ gas fired residential boiler can be easily converted to propane, which would be of similar advantage to houses in Alaska or California or British Columbia where earthquakes are more common; or in urban areas where the gas utility isn't reliable.

GV90+ Gas Boiler | Weil-McLain | Product Detail

Anyhow, it's possible that boilers made 20 years ago, and those that sold well in some parts of the continent were different than those available today. But, from what little snooping I've done on Weil McLain's web site I'm coming to the conclusion that being able to switch a boiler that's meant to fire gas to one that burns oil, or vice versa, isn't very common. If the boiler was designed to fire either fuel, you have that option, but if it wasn't designed for that, then I'm expecting that option isn't available to you.

But, I'm old enough to acknowledge that I don't know everything like I did when I was 15, so I might be dead wrong on this one too.