Depends on the gun. TM pistols usually have plastic slides and are best run on 'green' gas and should be fine even in colder weather. Red gas can be too strong and cause damage on plastic slides. I run me WE guns , 2 pistols with metal slides and 2 rifles , on 'red' all year round . Its stronger than green and suffers less at low temps. It gives a nice kick though and moves the heavier slides with no problems. Black is stronger again but ive never needed to use it , maybe in a Scottish winter ?
I use Nuprol gasses , but people tend to have their favourites.
You can also use propane , which is what green gas is, and I do on occasion if I am using the guns a lot in a day as it works out a bit cheaper in the long run. More knowledgeable folk than me can tell you more about propane use if you decide to try it.
