What o2's to get...
 

I've noticed my MPG getting a lil worse, and have 135k on the car right now, i dont know if the o2's are original or not, i have no codes, but im thinking they may be the cause, would cleaning them help (what should i use, carborator cleaner?) or should i just replace them? If so, which ones should i get?

If you're still on stock manifolds, just get factory replacements from GM. I've heard bad things about Bosches and other cheap aftermarket units.

Cleaning your o2's are a waste of time. They're a worthwhile investment of around $150.

AC Delco 100%!! NO AFTERMARKET! the bosch's internal voltage source wears down quickly. It is supposed to send a signal to the computer once they have warmed up.The aftermarket's drain easily.

The tuning section has a lot of information on O2 sensors.

If you want to clean your sensors, use a clean rag/towell and wipe them off. Do not use solvents.