I am not mobile but speaking from a client's point of view. I don't think I'd like to be charged extra on top of a treatment for petrol unless I lived quite a distance away. You would also have to make sure that you advertise the petrol charge so that it doesn't come up as a 'hidden' charge when you get there. When I did mobile treatments I just had a radius that I covered and included all my costs in my treatment prices.
Saying that, I tend to order takeaway from the other side of town as it is better than closer ones. They have no delivery charge up to a certain distance, then they charge a few pence per mile after that. I don't mind paying the extra as I know in advance and choose to use them.
I'm sure if you decide to add a petrol charge and advertise it so it doesn't come as a surprise, clients will understand. Failing that, you could up your prices slightly and explain that it is due to rising costs.