Gerald, gas is fine, but don't rub it in too much, this will eat away at the clear coat. The easiest way is to have your car properly waxed and then asphalt/tar comes off easily even with water and a rag.
I do clean my windshield with a gasoline soaked rag first, then wash the windshield, dry it and apply Rain-X. After that I wash the entire car and it is perfect. I do this to remove the oils and crap that other crappy cars on the road leave behind....on my windshied!