Dexron III or regular Mercon is the factory recommended fluid in any year 4R100.
Mercon-5 is more expensive due to it being semi-synthetic and it has one hell of a friction modifier package to keep torque converter clutch shudder at bay. Mercon-5 was originally invented to deal with TCC shudder in the AOD-E and 4R70W transmissions. It also has one hell of an anti-oxidant and anti-foaming additive package in it.
Due to Mercon-5 being a little bit more "slippery" than regular Dexron III/regular Mercon, you may notice a slight decrease in shift firmness when running it in your Dexron/Mercon spec trans without a shift kit in place. It will also make your torque converter clutch slide/slip a little more during its engagement, but this strategy of slipperier fluid keeps the TCC shudder at bay in these factory calibrated transmissions. Decent shift kits on the market address the slow factory application rate of the TCC in order to bring the TCC engagement on more abruptly to avoid the shudder in the first place. TCC apply will feel like another gearshift instead of the factory slide engagement since it would be applied quicker.
The only time I recommend Mercon-5 is in high rpm / high hp applications that throw a lot of fluid up out of the pan and into the tailhousing on hard launches. In instances like this, a lot of fluid is exposed to the rotating assembly at high rpm and the anti-foaming properties of the Mercon-5 are desirable. Mercon-5 helps noticeably in C-4s which historically have a problem of foaming trans fluid up out the vent in high hp applications.
Since the Lightning isn't normally a high rpm candidate, save your money and just run Dexron III/regular Mercon. Change it every 20-30k miles and keep it cool and you won't have any fluid related failures.
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