don't take my word on this, I've never tried it, but I've heard of people using wd-40 to remove tar and stuff from paint. Anyhow if it came down to doing any sort of sanding, light water sanding and rebuffing that side of the car would be your best option IMO
Edit: here right from the WD-40 website
I've heard that people use WD-40 as a cleaner/remover. What can it be used on?
WD-40 removes sap, tar, adhesives, labels and tape from surfaces without damaging existing paint. It's an effective cleaner for tools, equipment, and vehicles. Use it to remove splattered bugs from the front of cars. WD-40 will even help remove gum from carpet. Just spray, wait, and wipe with a clean cloth.
about half way down
http://www.wd40.com/faqs/