2009 4 cyl Galant transmission issue

Hi, new member here. I appreciate any advice available. A week ago I purchased an 09 4cyl Galant with 145K on it for my daughter. I noticed when driving at higher speeds above 45-50 if I accelerate I get a shudder from the front which feels like transmission slippage. I looked at the transmission fluid and its brown. Big mistake not looking at it before I bought it, but don't pile on me too hard, I'm already kicking myself. Obviously I want to change the fluid and filter but its hard to find info on that for an 09 and I've googled and you-tubed quite a bit. Is this a spin on external filter because that's all that Advance auto shows for it. Auto zone shows an internal filter AND a spin on type filter but they don't know the difference as far as my specific car requirement. I'm waiting for the chance to get it up on car ramps because its hard to see anything from the top, but it looks like the tranny pan is sitting vertical and facing the radiator is that right? My big question is if anyone thinks I'm wasting time doing the filter and fluid change since I'm already noticing the trans shudder at higher speeds? It seems absolutely fine and responsive at lower speeds and that's why I missed the problem when test driving because I only drove it around a housing development, not on the highway. Is there any others steps I can take that might help when I drain and clean the pan, ect.? Thanks again for any insight.

I would take a look at your transmission. You would know if theres a external filter. they are hard to hide and usually would be found on top. Theres a chance you may not have one. The earlier galants had them for sure 1999. You can find the internal filters on rock auto. A Rule of thumb is to put in the same amount of fluid that came out provided that fluid level is in the correct part of the dipstick while running and warmed up. I recommend sticking to mitsu auto fluid. Autozone sells a shudder additive for the transmission fluid, I used it on my 03 outlander. Maybe something you can test before changing the fluids.

Shudders at highway speeds could be suspension or wheel issues. I have never heard about a auto doing it at those speeds but I could be wrong. good luck im not on usually.

at that speed it could have been convertor lock-up shudder,,,did it help ??