Petrol is not a good idea.
It is probably the most efficient tar remover, but it also strips any wax on the car and will even desolve some of the coating of the paint. Defo a
Try a gentle tar remover. Assume Autoglym is good.
I got Zymol Strick on order for when I am in US next week.
As per Bro's suggestion, a good wax is the best protection afterwards. I'll put a few pics up of how the Merc and Scoob sometimes looked during winter. Simple spray off and it's all gone.
When using Zymol, don't forget to buff the thing again after a few hours. The wax needs to cure completely. If you don't, it remains fairly 'soft' (and will disappear more easily) plus it will leave fat-marks on the paint when you touch it (which is the problem Spike had on his Porker).