What octane should I use? Modded 5.0...

Just got an '88 lx w/ a 5.0. Engine mods include Cobra ported and polished heads (compression ratio?), upgraded injectors
(not sure how many #'s?), GT-40 upper and lower intake, Moroso cold-air intake, and Flowmaster exhaust. I just got it smogged and it failed the pretest. It had 15 degrees of timing , which they said should be more like 10 degrees. Turned out it had 2 bad O2 sensors. Is there a relationship between the timing and bad O2's or does the timing have to be advanced by someone? Anyway, I've been using 91 octane in Cali and I keep reading that everyone is using 87, and that's what the manual says you should use. Also, the engine was rebuilt 50,000 miles ago. So what octane should I use? Thanks.

you should use the lowest octane you can without causing pre-detonation. lower octane= more explosive.

I know that. But I would rather not find out the hard way. ;)

Turn down your timing to 10*, and run the 87. If you stay at 15*, your gonna need the 91.

Just got an '88 lx w/ a 5.0. Is there a relationship between the timing and bad O2's or does the timing have to be advanced by someone? Thanks.

No relationship at all. O2 sensors just go out over time.

Thanks for the replies.

Well, I looked over my smog sheets (which by the way I neglected to say the car did pass CA smog after the timing was lowered to 11* and a fuel system cleaning... So, I'm gonna go with 89 Chevron from now on. :)