I use a mix of flat and satin under the car and in the engine bay. I also use a can of Rustoleum in an aluminum color as a contrast and to help make the silver areas look better.
The issue I learned about using a glossy black paint is it makes it very difficult in poorly lit areas to spot any leaks/fluid that is easier to see on flat or with satin. I give the areas under the hood or under the car a good cleaning, dry the area off, scuff it up and use a degreaser so the paint will stick the best to these areas. Good luck !