The reason ethier works to assist starting a diesel engine is that is has a much lower flash point (lower than gasoline, even) and gives a couple seconds of heat to get cylinder temperature up to what is required to self-ignite the diesel fuel. For a diesel to start, the cylinder temperature must reach 287 C (or some number very close to that, diesel thereoy class was a few years back) to start. This is a compression-ignition engine, usually compression alone is more than enough to start one.

That said, your manual directly states to not ever use ethier with this engine setup. Sure, if you didn't have the heater grids in the air intake you might be able to get by with little trouble - I did on the farm with old International tractors. But as others have mentioned it's a big safety concern with these heater grids, which must not be working. I would start by looking at them. Just for referance, I'm in Illinois and have started to -5F with only the heater grids, and down to -20F with the block heater. I have not used a starting aid in the 6+ years I've owned this truck.

 

I think the grid heaters start to come on anytime below 59 degrees. But yes it should start with out them to a fairly low temp.

 

Diesels after time that will only start on either are what we use to call either dependent. What happens over time is the top fire ring becomes worn down and very thin, not holding any of the needed combustion during the firing off strokes. I saw this time and time again with the old detroits from multiple shots of the drug used for years in equipment. Eventually they run so poorly and slobber that its unbearable.