Mercon V was approved by Ford in a technical service bulletin in 2006 or 07 for all previous automatic transmissions and it's better than previous fluids that were approved in sequence from type F which I remember from back in the 1950s. Approvals step up with improvements in technology (formulations). Manufacturers who make the transmission fluids can make it to meet more than one vehicle manufacturer's specifications, thus you can see Dexron 3 and Mercon V for example on the same label and perhaps approved for several manufacturers.
Lets look at the IDI era and earlier. The reason for keeping transmission fluid temperatures below a certain temperature is that above that temperature the transmission fluid begins to break down, loses lubrication and hydraulic properties which will lead to transmission damage. I learned that from a former Ford transmissions engineer who posts at FTE.
As someone else here mentioned, Mercon V can sustain higher temperatures without breaking down. I don't know about those temperature specifics but I do know this much:
Full Synthetic transmission fluids can perform well at temperatures as much as 185 deg. F higher than non-synthetic fluids without breaking down and synthetic mixes also perform at higher temps than non-synthetic.
I used full synthetic Mercon V in my C-6. I also added an in-line MAGNEFINE filter (the real one made in Australia- lots of copies out there which have shown not to be as good - there may be an exception or two but I'm not taking chances at this stage. If you recall what is on the magnet in the pan and the black film on the bottom of the pan the last time you changed the fluid then you'll know why I think an inline magnetic filter (it has a filter medium as well as a magnet and a bypass as it starts to get plugged) it is a 'no-brainer' decision.
Just because the C6 normally works so very well and for so long with only a screen doesn't mean that adding a filter is not a very good idea to help it last longer. With the Full Synthetic fluid I BELIEVE that my C6 can work harder or at least longer at higher temperatures without risk from fluid breaking down. As far as I am aware the
main cause of automatic transmission failures in a good transmission is excessive heat breaking down the transmission fluid.
I also intend to add a couple of small very powerful rare earth magnets (neodymium) to the outside of the pan and I'll get those from inside of some old computer hard drives I have lying about (don't throw those away before taking the magnets out - they are fun to have with many uses) They most often look like small disk brake pads in the older hard drives:
The part with the holes is just steel that the magnets are stuck to and I leave those on to grab the edge of the steel to pull the magnet off of whatever it is stuck to - otherwise
the magnet is so powerful it is not easy to get it off!
I am going to also use a couple of these on my oil filter and on a coolant filter when I install one.
Back to Full Synthetic fluid being Mercon V in this case-
it also lasts longer maybe for the reasons mentioned above and/or other improvements in the formulation of the Mercon V specification whether synthetic or not.
That being said, I had installed a new and larger transmission fluid auxiliary cooler that I will probably rarely, if ever, actually need with a fluid that performs well at those significantly higher temperatures. I'll probably keep most of the cooler covered in normal driving conditions or all of it covered in cold winter weather and only uncover it in hot weather, (city driving gets transmission fluid hot) and when I am working the transmission hard, like pulling a load up long grades. I don't see any need (for me) to have a transmission temperature gauge, I can probably judge well enough by the engine temp gauge. I already have an EGT gauge. My truck weighs 6,080 lbs according to the DMV, with heavy duty factory options to pull an combined weight (that is with a trailer) of 14k lbs. and now has a larger aux. transmission cooler rated for 18k lbs. combined weight. I'm unlikely to ever be pulling more than 14k GCVW but I'll be able to.
Rare Earth magnets, in this post mentioning Neodymium magnets (alloy of neodymium, iron and boron) in various shapes and sizes
are very inexpensive on-line. You can get one to stick on the outside of your engine oil drain plug (even if it is already magnetized this will substantially improve it) for probably a dollar - I haven't looked in a long while. Use plated ones or coated ones for outside use to prevent corrosion of the magnet. computer hard drive magnets are already nickle plated.
I'd like to make a small magnetic 'sweeper' about 8 inches long by adhering these inside of a small and shallow rectangular extruded aluminum or PVC tube. I think they'll pull up a screw buried in the dirt where a regular iron magnet wont. They can lift thousands of times their own weight! In fact not much larger than this they can become dangerous (seriously!) They are a bit brittle compared to Iron, I think I broke one once.
If you want to know more and
intend to use them do read this for a start:
https://en.wikipedia.org/wiki/Neodymium_magnet#Properties
Sorry I wrote so much about these magnets but they are fascinating.
