can i use petrol as fuel in a rc nitro cars?

I take it petrol means gasoline and if it does the answer is absolutely not. The fuel you should use will be a nitromethane and lubricating oil mixture. some manufacturer's use castor oil some synthetics, and the it will have a % on the bottle that is the % of nitro in it. If you use anything but the nitro oil mix you will ruin your engine.
References :

You can use gasoline instead of nitro, but there is a modification that needs to be done to the engine. This modification includes parts that can only be found at expert rc shops and are a bit expensive. I suggest that you stick to nitro. However, if you decide to use gasoline, note that you have to add oil to it before it can be consumed by the engine. I think it's 5% for every ten litres…
References :
I 've been a modelist for 12 years.