2000 Olds Alero
 

I got an Alero GL V6 from my brother. He was told that the exhaust manifold is bad and that they'd have to drop the motor to replace the manifold. I smell no leak, I don't hear any hissing from either manifold - but the millage is lousy. I was thinking the O2 sensors might be bad but I'm looking for ideas that won't entail a mechanic because I'm pretty sure it's going to need a new transmission soon. None of the check lights are on.
Thoughts?

First, how many miles on the car? Tire pressures have a big affect on mileage, I run 35psi all around.
I would start with the basics, fuel filter, air filter, wires and plugs, injector cleaner, all things you can do by yourself, easy and cheap. The forward O2 sensor is the only one that has any effect on mileage, the aft one only tells the PCM if the cat is working. I've changed them before, it's about like changing a spark plug, no special skills required. Only get a Delco one, some folks have had problems with other brands.
Try those ideas and let us know how it's working out.

And if the manifold is cracked they sure the hell don't need to pull the motor to get the manifold off. Did they say if it was the front or the rear? Did they take the heat shields off the exhaust manifolds? If they didn't take the shields off I don't see how they could see if one was cracked. They only time I have seen cracked exhaust manifolds is when the car has had a bad headgasket for a while. The coolant hitting a hot manifold will crack them.

It's only got 87k miles on it.
First calculation for millage was about 19mpg but that was all in-city driving and that's supposed to be in-line with the EPA specs. So might not be as far off as he thought. Already changed the oil and air filter, checked tire pressure. I'll do plugs and wires this weekend, and I do Gumout cleaner every oil change in the fuel - should I get more aggressive than that?

Can't imagine that anything is wrong with exhaust - no puffy noises, no smell from the engine compartment. Pretty sure that was a rip-off (gotta love Pep Boys)

Trans slips when you're hovering around 2000 rpm (can't decide between overdrive and drive) and does wicked shudder-shifts if you're pushing the throttle. But if you drive like a grandpa- you wouldn't even know you had a problem.
Already changed the oil and air filter, checked tire pressure. I'll do plugs and wires this weekend, and I do Gumout cleaner every oil change in the fuel - should I get more aggressive than that?

Clean the throttle body out too.