Thanks for the replies.
It's as I suspected. A lot of chemicals promote the removal of oil stains but never do.
I've been avoiding drives with oil stains. The requests that I have had, I usually tell them its unlikely to completely come out and
basically try to talk myself out of the job.
From now on i'm going to let them know i'll do my best but no guarantees, but offer to replace the blocks. I just need to find out how to go
about replacing the blocks.