meguiars super degreaser is best, it can be used on engine undercarriage and interior, its supposed to be diluted according to your liking , for interior it needs to be heavily diluted, it comes in 1 gallon and it is pretty economical, use a degreaser that you can dilute with water, stp you can't dilute, ideally you should be using interior or upholstery cleaner, if on budget astonish has that