Best de-greaser for old caked-on oil/dirt?

Walmart Auto Department. Grab a gallon jug of Super Clean. Comes in a big purple jug. Not to be confused with Purple Power or Crud Cutter although they work too. You can dilute it like crazy and it still kicks ***. If you wash your hands with it straight it may never quit making soap! Buy a good quality spray bottle and dilute it to your needs.
Super Clean. One of the few snake oil products I endorse because I've had good luck with it.
Also Never Seize and Rain-X products.