Looking for something that will clean the 40yrs of black gunk on the underside of my hood, but not damage the paint.

Simple Green?

 

As good as any.

 

Should I dilute to preserve paint or no worries?

 

IMO, Simple Green is pretty mild. If you were using SuperClean, I'd worry. That said, 1:1 with hot water should probably do the trick, and the hot water should help penetration. If not, use it straight.

 

I used Fantastic spray cleaner on mine.

 

Unless your using a pressure washer, you probably wont get much by off hand if its really thick

 

I'd start this job with a pressure washer. If you don't have one, go to a self-serve car wash. Make sure you take along what you'll need to dry out the inside of the distributor cap, so you'll be able to go home.

 

Simple green and a scrub brush got the underside of my car looking like this.

 

WOW nice results!