Depends..............are you going to get the engine bay painted ? I'll assume not, so the best stuff for removing crap is Gunk. But beware, it's petroleum based so you won't be able to polish straight over the top. You are then going to need to T-Cut the whole lot, and that is bloody hard work..........trust me !!!

Wear latex gloves when using both, as Gunk is a solvent and T-Cut will make your hands smell for days.............top tip !!!!