Every few months I clean my French horn in an enamel bathtub filled with warm water and added dishwasher liquid. The oil and grease residues that attach to the detergent float to the water surface and collect and attach to the bathtub wall. After draining the water the residues leave an everlasting visible yellow-brownish ring that seems virtually impossible to remove. I tried to clean the stains again with dishwasher liquid, bathtub soap removing detergent, regular "general purpose" bathtub cleaning agent as well as another general purpose bathtub-cleaning agent marketed to be extra strong and work against "any type of stain".
I assume the regular bathroom/bathtub cleaning agents are not targeted towards the dishwasher/oil residue, and hence I'd like to ask if there is any effective remedy to remove such stains?
PS: The bathtub is enamel and despite being 25 years old still rather shiny.