I just got a used 2000 Bonneville SE and dont have the manual. I'm wondering if anyone would know what the recommended fuel grade is for the car. I put 89 in it and the service engine soon light came on but that might have been from the fuel left in the tank. I'm now close to empty and the light is off. What type of Gas should I be using and should I throw in a bottle of fuel system cleaner?

GM recommends 87 octane for the non-supercharged (naturally aspirated) engine. You may want to get that code scanned. It will be stored even if the light is off.

When you fill up, when the pump kicks off, do not "top off" the tank. That has been known to cause some issues, and could be the reason for the light.

I agree with Archon on both counts. Especially, don't "top off".

Yea, I forgot to mention I got the code checked and the very helpful Autozone employee said I used 'thin gas'

I did top-off, maybe that exasserbated the situation.

Thin gas?! That'* a new one. Did they give you the code?

You may also want to look at the gas cap seal. Figure out where the cap seal meets the metal top of the filler tube. I rub vaseline on both. Make sure the top of the metal tank filler doesn't have a coating of crud on it. That'* why I rub it to smooth it down for a good seal.

If the car runs the test to be sure the tank and fuel system is holding pressure and it finds the pressure (or vacuum) decays too quickly it turns on the SES light. It runs the test when started and the coolant temp is between a minimum and a maximum. As I recall my 98'* description it'* below 170 and above 120, e.g. For my car that meant driving 4 miles and then shutting it off to get coffee. When I restarted it the test runs and the light went off or on.

That'* a new one on me too "thin gas"? I think your AutoZone helper may have been smoking "thin gas".

If you go back to autozone and get the actual number of teh codes you have, we can more than likely help you fix it up real good.