Back to being serious, there is some spray on stuff that dries clear without being tacky, except I can't remember the name of it which we used to spray on undercarriage parts to prevent corrosion. Damn it, I'm drawing a blank. Anyways, I've used it on outdoor swings, barbeques, lighting fixtures and it is compatible with bare metal surfaces of all types. Maybe I can find it on Google.

But then most all engine surfaces and engine compartment parts are painted or clearcoated and what's not painted is usually non-corrosive. Cleaning the engine compartment with a good degreaser and hitting it with a detailing spray is probably good enough, so long as it's done periodically.

WD40 leaves a oily residue which in turn becomes a dirt magnet. If you wipe it, whatever's left will likely vaporize from engine heat. I think WD40 is rather pointless as a protectorant; it's value is only as lubricant.

Somebody told me that WD40 can remove gum from clothing, replacing the age old fix of using a stick of butter. Haven't had the occasion to try it yet.