I recently became a huge fan of Meguire's Hyper Lube - diluted down to a 3-1 ratio. I rinse the engine bay, spray on the HL and allow to dry and then wipe off any excess. The stuff gives a nice, like new appearance to all under-hood components without the Armorall greasyness. Just make sure the engine bay is clean prior to application.

Cheers,
KrzyFast