Due to leaky valve covers, the bottom of the hood of my butternut yellow Chevelle has quite a bit of oil/dirt grime on it. I've already cleaned off a lot of it, but there's a thick residue that won't come off with normal soap and water. I know that gas-WD40-brake cleaner works well but I don't want to apply anything that will take the paint off as well.

What would work well for cutting through the grime but not eat away at the paint?

I have never had WD-40 hurt paint. The best thing to use would be prep clean, this can be purchased at your local auto paint supply store