What glow plugs are you using?
Ive found the OS#8s to be great for allsorts of conditions and engines.

Regards to the temperature - personally, I dont like to run when its cold out. Besides my fingers getting too cold, its sometimes tricky to get the engine up to temperature and running/restarting reliably. If this happens then the engine will suffer from accelerated wear. An onboard temp gauge will give you a rough idea of temps, not ultra accurate, but a ball park figure at least.