My best guess would be a petroleum based product like Goo Gone. Comes in a spray bottle and sells in hardware stores and Wal-Mart. Make sure you wear gloves and keep it away from children and animals. Do not use a razor to scrape it because you may damage the finish on the siding.
Also - you may want to contact a store that specializes in siding and see what else they recommend. They may have a better product or method to remove the tar. Call them before you try Goo Gone.
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