I cut a piece of 2x4 wood about 4-5 in. long. I then removed the tight cap(think it was channel locks I used), then laid the cap on the wood and traced around the portion of the cap you are supposed to turn by hand with a pencil. I then took a wood carving knife and carved it out enough so the cap would fit into this area of the wooden block.
Keep the wooden block in the trunk so you always have it. It gets the cap off nice and easy every time. It seems these MB oil caps get this way over time, so I see no sense throwing money away on a new one when it's gonna do it again.
My 2 cents.