If you are netting $6,000 in credit card rebates then you are profiting from the business, right? The $300,000 in oil had a net cost to you of $294,000. If you tell the IRS that your business broke even (or otherwise do not report), you're concealing $6,000 in net income. I do not think that the fact that you yourself purchase more than $6,000 worth of oil enables you to argue that you're just earning an oversized discount on your portion of the transaction.
Here is an IRS comment mainly about the somewhat related issue of personal use of frequent flyer mileage earned on business travel:
Note specifically the following:
I don't see how you can possibly argue that what you're doing is other than a tax avoidance scheme: you yourself described it as a way of avoiding taxes.