The difference lies in the fineness and number of strands of wire. A high-grade automotive wire will have lots and lots of very fine strands, while a house-grade wire will have larger strands but fewer of them. The greater number of finer strands is better for automotive use as the wire will flex better, have a smaller bend radius, and be less prone to breaking strands due to fatigue from flexing.
That said I used high-grade 10AWG house wiring for my headlight harness and it has worked quite well so far.
Am I reading your post correctly that you want to use double-ought wiring for your radiator fan? That's ridiculously overkill, and given the diameter of 00 wire it would be a bear to work with. If it were my rig, I would pony up and buy some good 10AWG wire, and save the 00 for new battery ground terminals.
IIRC, a recent issue of JP Magazine had an article comparing various grades of wire. I don't see it on their website, but I can look when I get home if you are interested.