Try WD-40 first...that works well. But I accidentally found out that rear differential fluid worked well also. I got a little bit on the underside of the car when I was changing the differential fluid, and it got all the grime off the spot it spilled on! So I tried it on my plenum, and it worked great. It didn't harm the paint at all.
Tim 1990 300ZX Stage I Twin Turbo 1991 300ZX Stage III Twin Turbo (just added to Z family)