Diesel won't hurt your seal anymore than Kerosene!!!
Fuel 101 by Richmond Laney
The difference between Diesel Fuel and kerosene
The differences between Kerosene, jet fuel, #1 Fuel oil, #2 Fuel oil, heavier fuel oils, gasoline, etc. are the types of hydrocarbon chains that are separated and concentrated during refining and additives. For example gasoline has an octane rating which compares the gasoline blend with the performance of pure octane hydrocarbon with eight carbon atoms. Diesel has a septane rating, please note that septane has seven carbon atoms in its chain. Octane rating is how hard it is to ignite and septane rating how easy it is to ignite.
What this all means is that the heavier fuels have longer hydrocarbon chains, higher BTU content, are thicker and are more tolerant of contamination (also they are allowed to contain more sulfur).
K-1 Kerosene, #1 diesel, and jet fuel (JP4) are closely related to each other. #1 and JP4 have higher allowable sulfur than K-1 (kerosene is also called coal oil by old timers). Since #1 is a shorter hydrocarbon blend it has better solvent properties than #2. (We used to clean out our armored vehicle engine compartments with diesel fuel in the Army, also gasoline used to be sold as petroleum naphtha as a cleaning solvent before it became a motor fuel.) It also has a lower gel point, which means that it takes a colder temperature to turn it to a molasses like thickness. Because these fuels are blends they don't freeze like water, they act more like Jell-O slowly getting thicker until they set. This is why in cold climates it is suggested that #2 be blended with kerosene to keep it from gelling to soon. (Mercedes suggests using up to 30% gasoline in very cold climates).
In the US fuel oils are blended according to climate and location and there are also additives that control gel point, contamination (like water), change the flash point, to identify different uses (dyes) etc.
There is no real benefit to using kerosene /#1 over #2 unless you need to because of cold temperatures or you just want to clean out your system. If you switch to #1 please change your fuel filter to avoid clogging from any gunk removed. As it has been pointed out #2 has the higher heat (BTU) content.
All of this is good to know as in a pinch you can use diesel (or kerosene) in your furnace as a correspondent in Florida recently did during a cold spell when he couldn't get a delivery of fuel oil right away. (Conversely you can use fuel oil in your diesel car but the Feds don't like that as it is not taxed as motor fuel, Several times I have put motor oil in diesel engines when I have run out of fuel.) Personally I would use whatever is cheapest for that area, temperature range and manufacturers recommendation. This means #2 for the most part.