Engine oil and ATF fluid

[Big Ashy]

Any advice on this would be very helpful, and no, it wasn`t me!

Gets a call from my dad this afternoon who says that he has dropped a Bk and put engine oil in the power steering top up reservoir whilst attempting to put it in the motor.
I drive round there to see exactly what he has done and the reservoir is to the top. What used to be nice and pink is now a little on the grey side after putting approx 100ml of Shells best in.
I have managed to symphon off the contents of the reservoir, will this be suffice topped up with new ATF fluid or should the lot be drained and blown through. The car has not been started and for reference it is a 6 month old 535d Sport.
Cheers Adrian.

Edited by Big Ashy on Monday 29th January 23:19