Is powdered milk considered chalev stam?
This is a matter of debate, and one who is lenient has whom to rely on, especially if it is mixed into a product. However common custom has become to be stringent [for one who is stringent in chalav stam]. This is due to the fact that powdered chalav yisrael has become widely available.
See Shu’t Har Tzvi Y:D 103 who is lenient. cf. Chazon Ish Y:D 41:4 who is stringent.
See Tzitz Eliezer volume 16 siman 25, that when put into a mixture one may certainly be lenient, in his opinion.