In addition check that you have the requisite insurance especially third party requirements. Often this is payable in US Dollars so ensure that you know of it beforehand and have the necessary currency available. Similarly make sure of the currency limits and requirements of the country you intend to drive to. Toll roads are becoming more and more common and payment is mostly for cash. Fuel in South Africa is of high quality and unleaded fuel is just being introduced here.
Fuel is available everywhere and there are many well equipped rest station in the country.
Payment for fuel by credit card is only available to carriers of garage cards. These are issued by credit card companies to local clients. Credit card payments for fuel is not accepted. The further north you go the less dependable fuel supplies become. Zimbabwe is still fine.