You need professional legal advice. See another solicitor (I would suggest a different firm as well).
You need legal advice and it seems like the one you've got isn't doing his job. After all the solicitor is supposed to act on your behalf and do what you ask. Another solicitor may tell you, however, that you may not get extra money but they should still take the matter further if you ask them to - they have no choice; they act for you and have to do what you ask.
I don't know of the circumstances but if you are owed any futher money due to their negligence in the accident, they it may be the case that AIG don't owe you anything as it isn't covered in their insurance policy. If this the case, however, then Murco oil would owe you any outstanding compensation.
You could also report the solicitor to the Law Society who regulate solicitors if you are very unhappy with his actions.
As a final note, I thought it was the case that you are supposed to claim compensation from Murco Oil, as they are the party at fault. Murco Oil then claims this on their insurance. As far as I'm aware you don't claim directly from the insurance company. I could be wrong, however. But this is something worth mentioning to your solicitor.