HELP! i cant figure out the problem! (hard to accelerate up hills)

My friend and I both unfortunately have Automatic Aveo5 hatchbacks, both 4 cylinders, hers is a 2011. I have replaced the thermostat, coolant resivoir, spark plugs, valve cover gasket, did two coolant flushes and now I did a tranny flush and filter change. Her coolant still has oily sludge in her new resivoir. There is no coolant mixing with the oil though. When I checked her tranny fluid it looked like a light brownish yellow, and honestly to me it looked like there was a hint of green to me...kinda confused...maybe a bad oil cooler? But she isn't overheating or loosing coolant anymore since I replaced all that. So I flushed the tranny fluid last night and it was black, mind you there is less than 100k on it. I cleaned it all up put on the new filter filled it back up and now when I pull the dip stick out it looks darker.

Her car is still having the same main issue, its hard for her to accelerate up hills, the rpms stay normal but it almost feels like its struggling to climbing the hill and will also do this at higher freeway speeds. No white smoke, no leaking, no coolant in her oil, no overheating, no check engine light or codes, oil light is on, I was going to do an oil change today but I wanna figure this out before I do a bunch of stuff that may turn out to be pointless in the end.

I honestly really cant think of anything besides her tranny fluid being so dirty that its just not performing at 100%, worst case scenario, bad solenoid(s)?

Did you say "oil light is on"?

How long?

like a week, but I changed her oil yesterday and cleared the light, its just an oil change light, I had it on mine too but it hasn't come back on since I changed the oil. when she goes up a hill and drops its into 1st gear it doesn't crawl, but if she leaves it in drive it does and if she is on the freeway she obviously cant drop it to 1st and it will do it for her at high speeds...timing? her car is a little loud like its maybe the timing belt.. I also noticed her flex pipe is cracked about half way around.

I really hate these cars honestly we haven't been able to catch a break with them the past three weeks and its kinda getting old, once we think things are good something else happens.