I am sorry about your predicament here! You are a good owner to be trying to get this motor oil off your kitty.
You may be surprised to learn that motor oil is not very toxic to cats....I know there are reports all over the internet saying that it is, but ingested motor oil is not likely to cause anything more than loose stool or discolored stools.
The real danger lies in if he inhales any of the motor oil. Oil of any kind in the lungs is not very forgiving. If you notice any labored breathing, I would take him in. Inhalation can happen accidentally as he attempts to groom the rest off of himself.
If there is anyway that you can, I would repeat the bath using Dawn dish soap (or equivalent grease cutting dish soap), and copious rinsing.
So monitor his breathing and try to remove more of the oil, but if he has only ingested it, you are unlikely to see much of a problem.
Let me know if you have any questions.