Black marks on cuffs of shirts

[Gastonladybird]

A couple of dh shirts have black marks on cuffs (not sure what from it's where the double cuff turns back ). Marks are quite black (not just dirt) and normal wash with bio powder /vanish oxi isn't shifting even at 40 degree.

Any thoughts on what this is/how to remove?

My mum always rubs neat washing up liquid into Dad's cuffs before throwing them into a normal wash and that seems to remove most stains. Dad is a coach driver so his could include oil or anything. It always seems to work for her.

That's a good thought - thanks