Lowest safe starting temperature 580C

Hello, what is the lowest safe temperature I should attempt to start my Case 580C? I use my 580c for burying horses, snow removal or other part time uses in the winter. It is shredded but not in any heated building. It has no plug in heater for the coolant or oil. I do not attempt to start it in weather below about 10 degrees F. With a shot of starting fluid it has always started and I let it warm up with out revving the engine or moving it. I don't want to take the change of injuring anything with a cold weather start. The guys in my Antique Tractor Club need some logs for the sawmill lifted onto a trailer as soon as possible. I hate that rock crushing sound when I start my 1982 GMC dump truck W/6.2 in the cold weather.

Thanks for your thoughts, Kurt

Lowest safe starting temperature 580C