It could be a lot more expensive but I wonder if standard diesel would work very well.
Perhaps some of those who work in country areas can advise?
Farmers can use a heavy oil for their tractors which are not subject to tax and might sell you some. But again we need someone who can advise just what effect that would have on the boiler operation.
Wouldn't advise using diesel, whether red or otherwise, in an oil fired boiler, whether as a stop-gap or not. It won't burn as cleanly as kerosene, which could emit dangerous levels of CO, and could potentially foul components.
As an addition, and to answer your original post, you could use blue paraffin.
Pink or blue could be used, it is just a gimmick. As for how close the viscosities are, the reason 28 second oil is used in a boiler that requires it is because it should only use that type of oil. Using 35 second oil will make the boiler burn hotter, but also less cleanly.
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