Personally, I think the 15mm spacers are a better fit. I also have 18x8 wheels and they are ET38. I probably could have gotten away with 20mm, but i didnt want to get new bolts. You need to make sure you have exactly 15mm of thread left on the bolt when the bolt is inside the wheel. Too long and they will hit the ebrake components inside. I got my pair off ebay for $80 shipped overnight.