Purchase the cheapest fuel-'round here we call it "frog wizz." You will not only get the worst possible performance but you'll likely destroy pistons, rings, and cylinders. Your engine's manufacturer specifies a minimum octane rating. Go lower and your engine may knock or ping under load. It can put up with this for a while, but do it for too many tankfuls and carbon can build up in the piston ring's grooves and behind the rings. This forces ("jacks") the rings outward, scoring the cylinder or, worse, locking up the engine. While almost anything will run some of the smaller outboards (rated for as low as 84 octane), most engines require at least an honest 87 octane, with 89 preferred. For high-performance models, use 91 octane.