Diesel will start to change, depending on blend and variety, as high as around 40 degrees. At that temp, it can start to cloud as the wax particulates start to solidify... not a major problem, but still leaves you with tiny particulates that are going to be there even after you warm up the system. This isn't likely to cause any issue in your fuel system, as at this temperature as it is more likely to dissolve from engine heat if you properly warm up your engine and glow plugs. Gelling is the next stage beyond Clouding, where the next level of solidifying begins. At this point the actual fuel is starting to change and create heavier crystalline particles. This can happen as warm as 35 degrees without winterized fuel if the the particulate content is high enough. Suck enough of this stuff into your fuel lines and filter, and you are going to have problems. Think "clogged arteries" and "truck angioplasty". (It's a bigger problem with biodiesel, and since I try to run about 80% biodiesel I am getting a quick and dirty education on this right now as it has been about 38 here at night as of late and I've been dealing with some rough starts even at 44 degrees.)