I can´t say anything bad about Ford.
We also have a Lincoln Town Car and it is THE most reliable, easy and cheap to repair car I ever had. 150000 trouble free miles so far and I will not let this car go for a long time.
Never had been so satisfied.

Besides the Corvette and Cadillac, GM has not much to offer. As we also have a Cadillac, I can tell you compared to the Ford made Town Car it is a hell to repair. All squeezed in engine and such. But it rides nice though, more modern car than the Lincoln, with all its pros and cons....

 

In regards to GM reliability and durability, I purchased a 2007 Tahoe brand new and just sold it in Oct 2012. I sold it with 62,xxx miles. The only issue I ever had was the blower motor fior the front of the vehicle went bad. I changed it myself in 30 minutes. The only other expense was oil/filter changes and new brakes at all 4 corners at @ 55K miles. The brake pads wore a little early because I used it to tow.

 

I changed the fluid in the 4L60E's every 25-30k. I've never had one the fail before 175-200k. The one in my 07 has 127k on it now.

 

Sorry but a lot of talk about "modern" vehicles and fluids is not relevant to the OP's question. Yes it is interesting as we all have newer cars

but his/my Vette is 20 yrs old So I am reading this with the same interest.

Still not sure what the answer is though lol
Mine has 73k miles on it. I've owned it for 12 years and never changed it. I would figure it might be nice to have a transmission shop drop the pan, replace the filter, check/adj whatever they can see and refill. Less than 110 bucks for some peace of mind??

 

73K and 12 years - it needs and deserves just a dump, filter and refill. Do NOT consider a flush (my opinion). Rely on the shop since you're not doing it to use what's best for the application (their thoughts).

Come back and let us know what they recommended and used. How's that?

 

15K oil & filter in the auto trans is 4-5 times too often as necessary.

More so in consideration of the advancement of transmission oils over the years.

Dexron VI is so much better than Dexron III, I would not hesitate to do 50-60K oil/filter changes.

Truthfully, the fluid could go well beyond 100K, but as mentioned, filtration becomes an issue at some point, Trans filters of cars in this era do simply do not have the filtration capacity to do extended OCIs, and when you over-tax that capacity, you develop despots, and when you develop deposits, you kill valve bodies.

The other consideration not being mentioned is oxidation. Oil has a lifespan and a shelf-life by being subjected to atmosphere. It breaks down simply by sitting and by age.

Sealed transmissions in newer model year vehicles attempt to solve this problem, but older cars with a dip-stick invariably wil see some air inside the unit through the dipstick. Usually it's expelled when the unit pressurizes, but it's enough to cause the condition.

Regardless of mileage, you should change the trans oil every 5 years, which is the typical shelf-life of ATF.

Will your trans explode if you don't follow this strictly? That's unlikely, but it will contribute to increased wear at some level. How much is likely a topic more situite for a BITOG discussion, but what I will say is that if I pickup the C4 I'm looking at tomorrow, I'm going to change the fluid regardless of mileage or age. If you're on 12 year old fluid.. it's time to change.

 

There is considerable debate as to the backwards compatibility of Dexron VI for DexronIII, the debate will/can likely go on for years. The only thing that could prove the point is an established failure rate that could be directly related to the incompatibility of the sealing systems (all internal seals) of previous GM AT builds. Alison which was a GM division states specifically that VI is not compatible and specs out their own DexronIII/Allison 295 spec for their transmissions.

The sealed transmission isn't necessarily a bad idea as most try to point out. If the transmission is full and sealed there should be no concern until time for service unless there's a visible leak and that is the cause for concern and evaluation of the transmission at the time.

Overfill of an AT is the major cause of excessive heat and the oxidation or foaming of the fluid which can be directly related to failures. CMC/MOPAR has a specific temp that they want their transmission fluid level checked at with their specific dip stick.

AT fluid filtering, cooling and the use of the vehicle actually need to be considered when service intervals are discussed.

 

I think what adds to the confusion is that Dexron III/VI are both specifications and GM branded fluids (or in the case of Dexron III was as it is now superseded). The GM version of Dexron III used mineral basestock. Aftermarket options such as the two AMSOIL ATFs I listed above are synthetic basestock. In this case, there really isn't a longevity difference between AMSOIL's versions of Dexron VI and III as they both use synthetic basestock.

In my opinion, one of the main differences between the two is that Dexron VI's viscosity is notably lower than that of III, probably to improve GM's CAFE - and all the other manufacturers are going the same route.

When using good aftermarket fluids, I personally use AMSOIL Dexron III fluids for my vehicles that originally required Dexron III and only use AMSOIL's Dexron VI in applications that Dexron VI was the OE original recommendation.