I was just going to title this ATF but

so I am changing the fluid on my 2004 gmc 1500 Dextron III 140Kmiles

is there a preferred brand and product

normally with Oil I just use Blain's Farm and fleet 5w30 it is relabeled citco they also have a mercron dextron III at 3.99 a quart I assume citco re-labled it is in the same style bottle

or
is it worth it to go with something like a Castrol High Mileage that is double the price.

is it really twice as good or even noticeably better if I plan to change again at regular intervals ?

will only be getting about 1/2 of the fluid out 6-7 quarts then refill and in a few thousand miles repeat

I am not noticing any transmission issues I just think it is time to replace the fluid

The mercon/dex III will be just fine. I do not believe that the high mileage stuff provides any benefit.

Every 50K in my automatic transmission vehicles......jk

And save the old fluid for taking stuff apart and undercoating the truck!!.......jk

I knew what dextron was for, also knew what mercon was for.
When they labeled, dextron/mercon, I got confused.
Probably to make it "easier " for us lol.

Rural king might be a place to price it.

Price a new transmission, then make up your mind.

no Rural King around here Fam and Fleet has the re-labeled Citco I can get at 3.99 a quart , actual Citco there is 5.79 a quart.

most everything else is 8-10 a quart

besides Farm and Fleet the best price is to order from amazon at $25 a gallon

I have never had an issue with Farm and fleet oil or hydraulic fluid I was just wondering if there was a reason to go with the $$ stuff.

Farm and fleet is the only place I’ve ever bought trans fluid and have had good luck.I think you can replace dex 3 with dex 6 and you would be upgrading to synthetic.

Have you priced a regular service at trans shop?
One and done, converter flush, filter, and mainly, those cables stretch and they know how to adjust.
And other tricks.
Look up detent cable adjust.

My experience is with a later generation than yours.
There are Goodwrench guys on here, hope they chime in.

anything short of liquid gold would be cheaper than a new transmission

however if the store brand has the label that it meets or exceeds the original fluid the question is does the expensive name brand do anything special that the original wasn't doing.

Is this the first change in 140k ?
< Starting an ATF change war here , *flame suit on* >

Some transmissions don't do well if the ATF has not been changed in a long time. Sometimes changing the fluid may make things worse. You are borderline there. If it was 200k+ and never been changed , I would recommend to not change it. ATF should be changed regularly or not changed at all.

IMO Citgo is lower quality than most. Find a better product. Plus Citgo is owned by the gov't of Venezuela who hates the USA. Transmissions are crazy expensive to fix nowadays. Spend the money on better fluid.

I saw no advantage with the "high mileage" over the conventional oil, as far as consumption in an older engine.
As far as transmission fluid goes, stick to what the manufacturer recommends. I have also used Lucas "Trans Fix" as an additive. (not added to cure any problems).

don't know if it was changed before it was owned by the power and light company till 120K when I bought it .

it is an old surveyors truck fluid was nice and pink on the dip stick when I bought it

don't think it ever pulled a trailer till I put a hitch on it and my little boat isn't exactly a huge load
I had to add the wiring for trailer lights it had none

my utility trailer is a bit bigger but still a single axle and it gets very few miles with that some around town and a few firewood runs to the farm 7 miles away.

For what you are doing you are fine to use the store brand dex 3. Also nothing wrong with doing at a higher mileage. If you were having it flushed then you might be taking a risk of breaking loose deposits in the valve body and causing problems. But for simply draining it and re-filling you can do that no matter how high the mileage. Better put a new filter in while you're at it.

You can use anything that's approved by GM. Also, dexron VI is backwards compatible for any GM vehicle that originally had dexron III.

I'm glad to hear it was once a fleet vehicle. ( power&light co.)
Fleet vehicles have a better chance at regular maintenance than a homeowner owned vehicle. Good chance it may have had a transmission service before you got it. Pink color is good , however the 'sniff test' is another thing you can do. Hold the wet dipstick very close to your nose and smell it. It should smell oily and maybe a tiny , tiny bit acrid - normal. If it smells very acrid and burnt to the point you pull it away quickly , very bad.

I used to have a truck just like yours , '04 GMC 2500HD , 6.0L , 4wd , crew cab , swb.

Modern vehicles are using more and more expensive and exotic fluids as they are needed the way products are designed. I had to get some differential fluid for my truck. Only place that sold it was the dealership. It cost me $23 a quart.
Your truck may take different fluids as it may have a different engine , transmission , differential and payload capacity. My truck came equipped with the tow package and plow package , which means a lot of things were beefed up including the brakes and charging system.
Find the best fluids that meet or exceed factory requirements is the best way to avoid expensive transmission repairs IMO.

well I found some Valvoline at a better price than they wanted for it in town I ordered 3 gallons

not as cheap as the Citco / farm and fleet. but it is US made and a synthetic approved for all of my vehicles so one does them all.

I don't mind paying a little more to do business local but 40% more is too much

I would be better to toss the local store a 5 dollar bill to keep their lights on and order the 40% less exact same oil.

5 dollars more a gallon I would have bought it local no question but 16 dollars a gallon is just ridiculous.