It's an interesting theory but I've never noticed what I would expect to be a really obvious oil temp variation.
Without thinking very deeply about it, I think the theory might partly hold for an infinite-sink oil cooler i.e. one which instantly cools the oil down to the OAT regardless of anything. But oil coolers are not infinite; far from it, and if they were they would be useless because the oil would be far too cold to flow
