Be sure to scroll down... there may be more than one question on this page!
Oil and gas will also stain most other flooring materials you may use, too. If I were you, I would consider the advantages of a raised deck. It not only provides a solid surface for the shed but also reduces moisture within the shed... thus a little more protection for your tools! A raised deck will also give the shed a longer life regardless of the material it is made from... long term ground contact is not good for anything metal and most woods (except for treated woods, of course).
Use all pressure treated products for the deck, including the plywood. Basic deck construction is really easy and there are a zillion books on the topic. Plus if you have never built a deck before, it is a very satisfying project! If you decide to build it at ground level, keep the moisture down by putting a plastic vapor barrier under the deck.
Second, to protect the floor you can coat the plywood with a oil and gas-resistant paint. Better yet… if you expect a real mess… put a second "throw-away" layer of plywood over the first with just a few screws to keep it in place. When it gets too disgusting to bear, just take up the yucky plywood and install a new layer!