diff.

how big of difference would it be running 40 mg of dbol instead of 30

That's hard to measure. it would be a little better but I can't give you any figures as to weight gain either way...

depends on how many times youve used dbol before
if its your first time go with 30
hell just go 35 unless you are using 10 mg tabs

It's hard to say exactly........it depends on the person and how they respond. It shouldn't make too much of a difference though.