Yes Agree.If you read about ultrasonic cleaners, they require a aqueous solution to clean. Bubbles form on the surface of the object, explode/implode and that does the cleaning.
Oil will not allow that to happen.
An oil bath is good after coming out of the sonic cleaner.
I've put Remington 870 trigger groups in a sonic cleaner. I wasn't going to disassemble them, so it worked out pretty well.