In the shower? Why?
Back when I was young, cleaning greasy stuff like that with home made napalm was my go to method. I would grind ordinary bar soap with a cheese grater, then let it dry in the sun. Once it would pulverize easily, it was mixed with paint thinner until a jelly formed. This would stay in place on odd parts like valve covers, engine blocks and the like. After a day, the crud washed off with a garden sprayer. And killed grass.
yes, it burns like real napalm. Gasoline also works but evaporates faster. Back then, gasoline had tetrylethyl lead so we didn't use it for cleaning.