Ok, I am sure this has been discussed, I just can't find exactly what I am looking for?

I know that 90 535I factory Rim offset I believe is 20MM, I have seen a lot of rims that are 15mm. I have a nice set of rims that I am looking at that the guy claims he had on his 5er and from the back he read ET30, of which I believe is 30mm offset...I thought that 3 series are 35MM + for offset? Would 30mm offset work? Is that better or worse than 20MM....
Any advise would be great...if it matters these rims are 7.5" wide...
Thanks...