I’ve been cleaning up an old, filthy 35lb Toastmaster that we’ve been given. I’ve boiled out with a product called Array several times, There was lots of hardend oil residue that you couldn’t budge with a butter knife or score the stainless badly trying to get it off. I bought some spray called Carbon Off for the bottom of pans that’s stainless and aluminum safe. I found that a lot of that very hard, globulized oil would blister by the next day and easily scrapped of with a plastic spatuala. It will take an few applications, but it gets the job done without having to resort to abrasives. It’s not cheap at foodservice outlets ~ $10 for a large can, but works.