When I worked in a boiler plant, whenever it got very cold we would get the call to switch over to #6 fuel which I think is the same as bunker C. Nasty stuff, it had to be heated before it would flow, much less burn. Every time we got the call, a guy who worked midnight to eight would call in sick, every single time as long as I worked there which was 11 years in all.
It’s very rare anymore to find a boiler plant that does not run on natural gas. I inspect boilers for my job and I haven’t seen an oil plant in many years.