The general rule of thumb with plugs, weather they are turbo or standard is this......
The colder the weather is, the hotter the plug you should use and vice-versa too..........
The hotter the weather is your running the car/motor in, the colder the plug you should use.

If your running in anywhere near feezing temps........if your as mad as I used to be ...lol. Then go very hot! A 4 or even a 3 plug! (I've heard some have had good success with running the 3 even when it's slightley warmer too (40's !)
Then go to 5 and 6 plugs as you get up to 50 degrees or so and so on up the temp sclae.

When it gets stupid hot, over 90 degrees and above. You should be looking at 7 and 8 plugs! They are very cold plugs.

One thing to remember. Hot plugs 9for the conditions/ temps your driving in, help keep a good steady idle! The colder you go with the plugs, you may see idle problems!

This is by no means the end of the "which plug to use" story. But this temp indication will get you very close to the ideal plug you need for the conditions your going to be driving in.

Hope this all helps.

Regards,

British Menace