My old favourites, Gunk or Jizer, seem to have lost their superpowers, I think some ingredients have changed.
I have had good results with paraffin (Kerosene), or petrol mixed with diesel fuel, obviously don't smoke etc. washed off with detergent, Dissolve a couple of dishwasher tablets in a bucket of hot water, that works well.
If you need to clean a lot of engine parts, use an old dishwasher with extra tablets in it, (not SWMBO's). I use some stuff at work on the plant, it is milky coloured and smells of Brasso, I am trying to find out what make it is, but it ruins paintwork.