question about oil weight numbers

[keakar]

im trying to get an educated answer on this question.

is 0w-30w oil the same as straight 30w oil or is it different?

I am getting opinions that it is and that it isn't. I thought 0w just means its straight 30w oil but I have been told (by people I don't consider educated about oil types) that there is a 0w oil and 0w-30w is a mixed blend that shouldn't be used if the engine calls for straight 30w oil

on a side note, if its an older mower should I consider using 40w oil since engines get so hot in the summer around here or would that be a mistake? the mower still has great compression and doesn't smoke or use oil.