The main difference I've seen between plane and heli fuels is that most heli fuels have only synthetic oil, while most plane fuels typically have a little bit of castor as well.

As far as running plane/heli fuel in cars, I'd suggest against it. It will work, but you won't get very good performance. I ran my nitro buggy on 15% plane fuel initially, but it ran much better on 20% car fuel. The additional nitro no doubt helped, but the big difference was the reduced oil content of the car fuel. Plane fuel smoked pretty badly in the buggy, and top speed suffered pretty badly as well.