The 8L45 transmission has been a problem throughout the application range. You could also do some reading over on coloradofans.com where you'll find lots of content on this same slushbox. They put it behind both the gas and diesel versions starting about 2016-2017.

It is critical that the fluid gets changed out for the M1 blue label, and changed regularly. If the TC is already hosed/shuddering some of those can be cured with new fluid, some need to be replaced. If the TC comes apart it tends to send bits and pieces thru the whole trans and kills it.

I have a 6L50 - the earlier 6-speed also found in Cadillac and Camaro, in my 2015 Colorado 4x4. It also experienced some shuddering starting about 30K miles, which was cured with a Dex-VI fluid change. Still doing well at 50K miles now.

I have not had the opportunity to drive an 8-speed, however there are plenty of complaints about driveability with the 6speed, mostly it just wants to shift all the way up as soon as possible and lock the TC. Which is fine on the freeway, but for surface streets when you're coasting along at 45 and press on the gas it has to shift down two or three gears - which is beyond annoying. The solution tends to be to drop into manual mode so it can't shift all the way up. With a shuddering TC it's even worse - a fluid change makes a dramatic improvement in drivability. Clearing the learning points also tends to reset it for improvement for at least a short time.

I had been shopping for a CTS second gen and basically decided to avoid the '14 for its engine problems, and figured any of them would need a transmission fluid change. Seeing an XTS alongside a CTS more or less cinched it for me, switched my search to a XTS. I was half expecting the 6T70 in the XTS to behave like the 6L50 in my Colorado, but no, it shifts flawlessly, seamlessly, always in the right gear, just perfect. So now I know GM can build a proper shifting automatic. Still, they are well known for cooking the fluid, and when/if that happens they will eat themselves from within. I did a fluid drain at 60K and was shocked at how dark the fluid was. Evidently this is common, also found in the Impala and Traverse, as well as a plethora of other GM FWD cars. If you follow the 97K mile service interval recommendation you're more than likely to have a failed box by then. Some go longer, many don't - they fail like crazy.

Based on everything I've read, I cannot really recommend for or against a car with the 8 speed, but nearly anyone with some knowledge and experience with GM cars will always tell you to buy as new as possible with as low of miles as possible, and take care of it. Don't buy someone else's problem-car. Even better, get one still under factory warranty.

From wiki, 8L45 applications:

 

I too had similar concerns when we bought our '18 Prem Lux., but there are other factors that also need to be weighted...

Personally, I don't think I'd be comfortable buying a used vehicle across the country, or I couldn't see extensively in person, or test drive...but that is just me. There is alot trust being put on that transaction...

We were lucky. Multiple things were in our favor - and I realize, this may not happen for everyone. Based on these, I felt ok with the purchase of an '18. I'm sure others may state otherwise, but again, this worked - and has continued to work for us...

We purchased ours about a year and half ago from a GMC dealer that is approx. 1 mile from our house. We have really good history with their sales and service depts. Ours was a local, 1 previous owner, had low mileage (24K), clean carfax, interior / exterior color the wife wanted, very, very clean car. After an extensive test drive, one of the questions I asked was if there had been any tranny issues? Had the fluid been changed? TC replaced? Etc?. They called up the history and said, no issues reported / no service or repairs. So at least I have those facts. The service manager also stated if we experience any of the known 8 speed tranny issues (obviously they were well aware), don't hesitate - bring it in.
These factored into our decision...
We went home and thought about about...for one, we didn't need the vehicle...we have good history with the dealership, she really likes the car, it is low miles and is still covered by the factory warranty, the dealer is aware of the tranny issues and is prepared to fix it if needed.
We obviously bought it - went back the next day - haggled for awhile - walked away (again, we didn't need it) - played the game a little bit, and eventually settled on the price we were happy with...

A few months ago, the tranny started acting up. Harsh 1-2 up and downshifts (more when cold) and an intermittent shutter / shake a freeway speeds. We took it to the dealer, they performed the fluid swap / flush per the TSB. No issues since. The service manager also stated if these issues come back, or get worse, we'll replace the TC.
Again, our situation worked for us...we are very happy with the purchase, the car, our dealership, and the service we have received at the dealership.

 

Another thread on it:

GM 8-speed transmission

From GM Authority: The GM 8-Speed Transmission Needs To Go Away ASAP: Opinion BY JONATHAN LOPEZ — JUN 18, 2019 It’s no secret that the GM 8-speed transmission is not great. Also known as the 8L90 and 8L45, the 8-speed pairs to rear-wheel-drive architectures in GM’s bread-and-butter...

www.cadillacforums.com

GM has always had their issues, but try to keep the rigs going. Most of the dealers are solid.
I had a duramax with bad injectors (ext-warr) and a workhorse RV chassis with bad Bosch calipers (recalled).
The entire CUE dash has been replaced on my XTS. 🙄

I think I’ve owned every brand of car…the worst is Porsche and BMW - two brands you just never want to own out of warranty. LOL

 

I have no idea where GM's mind was with sticking to the 8 speed and all it's issues. This is almost the Northstar engine problem all over again and you would think they wouldn't want to relive that nightmare. What blows my mind is GM has all these test vehicles with this transmission in it and you're telling me not one of these guys felt the shuddering and harsh shifting and if they did, nobody thought to listen and tear it apart to fix the issue? Who ever in upper management ignored the feedback and pushed forward with this transmission should be fired if they haven't been.

With that said I have the 8 speed in my Escalade and while it does shudder and sucks at finding gears, once it warms up things improve and it's not as bad. It will never match the 10 speed which seems to be buttery smooth, knows what gear to be in, and it way better, but if you can live through that initial few miles of fluids warming up and reaching optimal performance, then it's worth having.

Definitely get an extended warranty if you plan to get this car with the 8 speed and stay away from a salvage title since this class action is picking up steam and you would hate to lose out on whatever the resolution is because of a salvage title.

 

Definitely get an extended warranty if you plan to get this car with the 8 speed

Cheers to that!!! 🍻

I knew from the get go I'd be purchasing a GMPP Extended Warranty for this car. Years ago, I was never a fan of these. It was like throwing money away. But I think they are definitely worth it for todays vehicles - especially one with known issues. The GM warranty must be purchased before the factory bumper to bumper expires. For us, I knew mileage wouldn't be our issue (since it's a low mileage vehicle), so I had to purchase it based on when the car initially went in service from the previous owner (prior to the 4 years). I looked at all the time / mileage's / price's that were available and chose the one that best suited our needs and purchased it from our selling dealer a couple months before the 4 year in-service date. And don't forget, these warranties are negotiable...especially if you pay cash. I was able haggle the price down even lower than quotes I received from much larger market dealers. Our dealer probably doesn't know that..., but they still made money...

 

I owned two 2017 CT6's with the 8L45, each one was returned to GM and the '19 replacement has been OK. Extensive reports can be found throughout the internet.

 

No doubt I can find reports of trouble out there… my question is how many of those 8L45’s had no problems at all? I realize there’s no good way to answer that question, since non-problems don’t leave a trail of data.

The fact that you had two different models returned for transmission issues is pretty telling though…

 

Clearly, after all these years, one has to conclude it was a flawed design. Mechanically, that would affect ALL of them, hence there are NO good ones. Consider all the owners who were told by dealers that they 'perform as designed' simply never went to the internet to share frustrations. Those that say 'never been a problem' perhaps have adjusted how they accelerate and coast in order to avoid the transmission's bad behavior. An then there are owners who never really did anything about it and figured it was just the way these fuel economy designs buck, throb, chatter and clunk. In other words, they suck.

 

My first 17 was so bad during its first 800 miles that I took it back to the dealership and asked if the dealership owner could leave his Escalade at the dealership for a weekend and struggle along with my gorgeous brand new CT6. It was SO bad for him I never saw that car again and they gave me another one. THAT one went for 4000 miles and then THAT transmission failed. They suck.

 

Anyone got a part number for the correct M1 "blue label" as it seems when I search that, all I see are black or silver labels. Can't seem to figure out the correct one. I've got a 2018 Camaro and I am thinking of flushing the fluid before any problems creep up (it's a summer-only convertible, so very low miles thus far).

Thanks
Max

EDIT... wait, I may have found it Canada Mobil 1... is this it (has some blue on label)?
Mobil 1™ Synthetic LV ATF HP

This is the old/wrong one correct (black label)?
Mobil 1™ Synthetic ATF

Thx
Max

EDIT 2 DAMMIT
Found this on US Mobil website via Google Mobil 1™ Synthetic LV ATF HP but shows black label!

BUT the primary Trans Fluids page doesn't show it listed, only the older non-LV fluid is shown.
Automatic Transmission Fluids | Mobil™
Mobil 1™ Synthetic ATF oil | Mobil™ Motor Oils

What the hell?!
Max

 

Evidently there's still some old black-label stuff floating around - but it has been discontinued and replaced with the blue-label fluid.
Simply search for: mobile 1 synthetic lv atf hp
Lots of hits, lots of sources. Sorry, can't help you with product to Canada.

But this IS the stuff you want, and if the car is still under warranty your dealer will replace the fluid for you under the TSB. Just tell them you're getting some shuddering and they'll change it.
Or you can do it yourself with a couple of drain and fills. It's kind of a PITA though, and you need an extractor. The car is more difficult since it's so low to the ground. Piece of cake on the pickup.
I parked over the driveway apron leaving LOTS of room under the trans to set the fluid level.

 

I don't think anyone has talked about how symptoms of the problem actually present themselves. The major issue is the shift from 1st to 2nd gear AT LOW SPEEDS. You're going to feel lit in parking lots and bumper to bumper traffic. If you accelerate spiritedly from a stop sign you're not going to see it. When you do feel it the actual feedback you get is almost like someone is stepping on the clutch, you hear the RPM's go up and then DROPS the clutch causing you to BANG into second gear. I got my 2017 and barely lasted a month with it before taking it back. It was under certified pre-owned warranty so the fix was covered. They did two things, swapped the fluid, which the previous owner did not have done and also changed the temp sensor in the transmission. This made the car run great for a while. I have had the car for 2 years now. Every once in a while you'll get a hard BANG into 2nd gear at low speeds but it does not present as bad as it was before the work I had done. Also it does not happen very often AND I SWEAR if you throw it into snow and ice mode and drive around for a while and then back to tour or sport it "goes away" I don't know if snow and ice changes the shift points or what or if something is binding and unbinding but I swear it does something. I spend a lot of time on 95 in the Bronx a lot of bumper to bumper driving and if it bangs into 2nd twice I just throw it into snow/ice drive for a few minutes then pop out of it and wallapooof its fine. These are the crazy things you do when you have this 8 speed...... Now can you shift it manually? ABSOLUTELY but let the RPM's climb higher in 1st gear before you shit to second, that seems to be the most fool proof method to get a good shift. The other thing to consider is I HAVE HAD NO PROBLEMS AT ALL at highway speeds. On the road once you are moving its actually been fantastic. Like I said I drive in a lot of heavy traffic and when the interstate is crowded and going 70 mph and you need to make precise passing maneuvers and you KNOW you need that transmission to shift EXACTLY when you stomp on the pedal so you can ya know cut that jerk off, I mean safely change lanes.... it has never failed me. To the point that I don't even bother with the paddles anymore. I only touch them when I get bored. So it comes down to your wallet and your brain machine. If you can afford the 19.... I would get the 19...... but if you don't mind a transmission with a......... lets say personality...... you'll be ok.

 

The 6L50 in my Colorado shifts all the way up to sixth when you coast at low speed, then lugs when you apply light throttle. Using manual mode keeps it from going past the gear you select, so 3, 4, or 5 depending on surface street speeds. No paddles on the little truck. Kind of a hassle, but at least do-able. Same deal - on the freeway it's fine. Heavy towing it's manual in 5 to keep it from shifting to 6th, about 2500rpms at 60mph. Otherwise it shifts back and forth for every slight incline, like overpasses and such. And otherwise the truck has been outstanding - it's a 2015 I bought new, 50K miles on it.

The biggest issue with both slush-boxes is the shuddering - I'm assuming that comes from the torque converter, likely the clutch pack. New correct fluid seems to cure that unless the thing is already hosed.

As I mentioned, the 2013 XTS with the 6T70 shifts perfectly all the time. I don't know why GM has such a time with these newer automatics.
I think I would try to avoid a 8L45/8L90 in any GM vehicle.

Here's an interesting write up from an independent transmission shop:
GM 8 Speed Transmissions | Advanced Transmission Center

 

Yes its that bad. They had the amazing aisin in the V sport they could have used but wanted to be typical GM cheap and use the Chevrolet 8 speed instead. Theres a class action lawsuit from truck owners for that same transmission.

My v sport aisin 8 speed is better than my ct6 10 speeds.

 

STUPID question, because of the computer machines and fancy bits you can't just throw that Asin transmission in the CT6 right??? like assuming it fit and stuff.

 

Two months ago I had the transmission fluid changed in my 2017 CT6 at 42K miles per the procedure outlined in the GM Service Bulletin. The shuddering went away. Unfortunately, about 500 miles later the thumping started. Everything worked fine while the engine was below its normal operating temperature of @210 degrees F. However, once the engine heated up the thumping started and occurred both while accelerating and decelerating. It felt like the transmission was slipping and missing gear engagement at every gear change below 50 mph. I had the cadillac service department check it out and they advised that the transfer case was slipping. They also advised that their experience was that the transmission oil change rarely resulted in a permanent fix of the shuddering or thumping problem. Most customers returned after their initial transmission oil change to have the transfer case changed. The GM service bulletin should be updated to address replacement of the transfer case first. Since the replacement, my CT6 is running smooth as silk -- like new. It is again such a joy to drive! It is very likely that your vehicle's transmission may still be under warranty so your out-of-pocket cost could be greatly reduced.