Automatic, what type, can you use type F, 120,000 miles starting to slip a little when starting out, only when cold. Going to try filter and fluid. Thanks

Dextron/Mercon

If you replace neglected fluid once it starts to slip, the trans will get way worse. If you are trying to get away with it as long as possible, you are better off leaving it alone.

^^^^^ Listen to that guy.
Do not change it now...it will certainly go ahead and die. The time to change your oil is not after the engine starts knocking.

Instead of changing it I would try some Lucas Additive. Good luck!

I'm interested in the theory on why a new filter and fluid would cause a problem.

The trans already has an issue. New fluid is very detergent , in most cases it will worsen the problem.
Like changing your engine oil after the motor is knocking.

Oil changes and trans fluid and filter changes are maintenance , not a fix.

Not sure why (but I do remember hearing it had to do with the detergents in the fresh fluid) but its definitely true. I managed an auto parts store for years, I've seen it happen probably a dozen times.

Not mine, my neighbor just got it. A older transmission guy years ago told me a old transmission with old fluid has grit in the fluid,that kept the car moving. Take it out put fresh fluid in, then it won`t move. that's why I thought about type F fluid. But anyway we already changed it and used mercon fluid and it seems ok so far. not a lot of junk in the pan, maybe just a dirty filter ,I guess time will tell

New fluid would have increased lubricity over old thus aggravate slipping. I'm not so sure trans fluid has detergent. Detergent is used primarily to suspend combustion contamination with HDEO a lot more than passenger. You don't use detergent motor oil in an air compressor why would you use detergent in a transmission?

My brothers Cavalier is pushing 300K Never had trans fuid changed. He finally changed the 4 spark plugs around 260K Lucky if it gets the oil changed 7-10K miles.