Bought a 95 Buick Riviera from a small dealer in North Carolina. This car came from Virginia and the CarFax said it passed an emission test every year until its last check in Oct 2006 at 42,000 miles. It now has 45,000 and the test drive went well. On the drive home the check engine light came on with no performance loss or rough idle or rotten egg smell from the exhaust. When you shut it off the check light does not come on. Sometimes will come on after a few miles and other times I have gone 40 miles and it never came on.
I went to 2 AutoZone stores to let them scan for a code but at both places an elderly person was using the scanner and both places could not get the scanner to communicate through the aldl connector. (I suspect they needed a different connector)
I went to another shop and I don't know how the fella said got a code about the EGR valve. They said $60 to clean the ERG valve. Seems to me if it were and ERG problem the light would be on right away. I talked to the original owner and although it has a supercharged engine he never ran anything but regular unleaded gas. Would you go with the ERG cleaning for $60 or try something else?
Could be the EGR, could be an O2 sensor, could be the gas cap if you just filled up. Someone got a code so that is more reliable than speculation from someone reading a newsgroup.
You don't trust elderly store clerks, you don't trust an independent shop, I'd have to suggest you take the car to an authorized Buick dealer and have them do whatever work is needed to bring the 12 year old car up to like new specifications.
EGR only works when needed. EGR carboning up is normal. Post the codes here and we can give you a few ideas of what would set those codes.
Cheapen out and go somewhere and pay to have the codes read. That's the only way to tell what the SES light is saying. Throwing parts at the car isn't going to do anything but cost you money. As for the octane, I have the same engine as you do and it clearly states 91 or higher. If you are using 87, and your knock sensor isn't up to snuff, you're not going to be driving that car very long.
Lots of '95 model gm's(Z-28's, 'Vettes, TransAms, ...) had OBDII systems, but still used OBDI aldl-connector for scanning. Scanners used a special connector-cable just for this 'hermaphroditic' (My own adjective, but it sounded good to me!) connector. s
Shop cleaned the valve but about 5 miles away the light came back on. They said drive it for a few days to clean things out. It does seem a bit better as it had a slight miss at steady speeds below 60mph and now its almost gone. There is no specific pattern to make it come on. Sometimes it comes on in less then a mile and sometimes I have gone 40 miles and it never came on.