I have a perfectly good chip fryer that is coated with caked-on old sunflower oil.
It's sufficiently hard and thick that the detergents I've used haven't touched it.
In the end I have put it all in the dishwasher and put it through a few washes. Some of the crud has become emulsified and almost peels off like latex. But there's a lot more to go and don't relish the thought of another dozen or more washes in the dishwasher!!
Are there any detergents which successfully dissolve this type of stuff?