I heat my house with oil, a truck comes every couple of months and fills a massive tank in my back yard.
This "oil" is basically diesel. It smells and feels identical to diesel. But it's about 70 cents cheaper per litre compared to road diesel. It's dyed red, and you are not supposed to put it in your car, but I reckon it'll be more than fine for older diesel engines.
The red diesel is not taxed like road diesel, and is much cheaper.
> I reckon it'll be more than fine for older diesel engines
There's always the risk of getting your fuel tank dipped if you're on road. Moreso for trucks, but some jurisdictions will set up inspections and check for dyed fuel and tear you an absolute new one when they catch it.
The exit of off road events is a common place to check this. So much so that there is a reputation in the off road community and now they don't even need to check often anymore since nobody is stupid enough to risk driving a truck that has ever had off road fuel in it there.
Here, that's commonly called red diesel (despite them changing to green decades ago) and it's sold for agricultural use. There are a number of cross border smuggling operations where criminals remove the dye and resell it for somewhere between the two prices.
Though primarily done to trucks, there are occasional fuel tests done by police. Even if your tank is currently clean, they'll occasionally pull out the fuel filters and check those for dye.