D3 is the form that your body can utilize easier. There is more than one type of supplement for Vitamin D. My Dr. told me to always check for the D3.
I don’t think that it matters what the temp is - other than you don’t want to freeze. About 20 minutes in the sun with as much skin exposed as decency allows and your body will tolerate is generally sufficient.
Some people assert that the reduction in the sun during winter contributes to the seasonal aspect of the flu/cold season.
My Dr. told me that in Alaska a lot of people have a “sun” room area - full spectrum lights to make up for the long time of darkness.
thanks very much you remain a wealth of knowledge.