87, the regular gas.

I use the 10% ethanol, allegedly 89 octane, because it is a few cents cheaper and seems to work just as well. Come wintertime I'll switch to regular for winter storage. Some say the E-10 gets worse mileage, but I have never been able to clearly measure the difference whether in car or motorcycle. As far as performance, I cannot tell the difference between regular 87 octane and E-10 89 octane.

I use 100% gas, better mileage and better for cab and gas tank,My Suzuki manual on my 1500 says don't use the 10% unless thats all you can get.I don't have the Suzuki manual on the S40 but I'm sure it says the same.

I use 93 as that's what the previous owner said he used. Would it be beneficial possibly to switch to a LOWER octane? I'm not sure if the 93 octane offers any perks for this bike.. and obviously if I can pay less for something it will run more smoothly on then it's an obvious choice and I'll make the conversion next time at the pump.

I think the 87 regular is just fine but around here it has 10% ethanol, Ive been running 89 octane which seems (to me) to give better performance with less stumbling.  I don't know if it has as much ethanol or just what.

BTW, looks like the feds will be allowing up to 15% ethanol now.
I think that a small percentage does offer some benefits but after that it is detrimental to performance and mileage.

I am not a chemist, but I had the opportunity to ask one about octane and its effect on motor fuel. He told me that higher octane does not necessarily burn cooler or result in less deposit. Higher octane, he said, tends to suppress pre-ignition, so that the engine does not "ping," which means the fuel does not fire in the engine before it is supposed to fire. Higher compression engines need higher octane fuel because they tend to pre-ignite. These lil Suzys are not high compression engines, and the owner's manual specifies 87 octane fuel. I run 89, simply because it seems to my ear to run a little better.

Engine is designed for 87 octane. That's what the engineers say. 87 octane delivers more energy per unit than higher octane grades, so if you use it you'll get higher miles-per-gallon and therefore cheaper operating costs.  Higher grades exist to prevent pre-ignition (pinging) in high-compression engines. Stock Savage/S40 certainly does not have a high-compression engine.

If you can find 100% gas (no ethanol) use it. Some of us do not have that option.

Shell advertises the same detergents in all grades. I use Shell when I can (it's a top-tier gas, see http://www.toptiergas.com). There's no cleaning advantage to go with grades higher than the called-for 87 octane.