[cwebster]

Originally Posted by TJay74

I will have to go find some of that then. As you said, while the RP MAT ATF may not be what GM wants you to use, the immense improvement it made in shifting was a night and day difference in shifting.

When I first got my 1LE and left for work on a 35° morning it was 6 miles before I could get the shifts to go without it sounding like a bag of marbles where in the transmission. The 1st to 2nd shift was so brutal and rough that I was surprised GM QC was ok with this kind of shifting effort.

Neither my 2001 Formula Firebird with the M6 nor my 2008 Corvette with the M6 shifted like this, both were smooth as butter.

Wow! That sounds horrible! I had basically the same complaints in my 5th Gen 1LE. In cold climates machines need lower viscosity fluids if you're going to drive them before they warm up. Suppliers often achieve this using "friction and viscosity modifiers" and "film strength" additives. My recommendation, regardless of the fluid you use, is to let the car warm up a bit more before driving when it's cold.

Since both the 6th Gen SS 1LE and ZL1 use the same M6 (not sure about the non-1LE SS), you'll probably be okay with the Max ATF if you've already swapped it in, since Tremec calls for ATF. If not, I'd go with the Synchromax.

Main reasons I chose Synchromax over Max ATF was (1) Synchromax was recommended for the GM fluid replacement in their cross-reference chart, and (2) disclaimers listed in the Max ATF spec sheet cited GM Dextron VI, the ATF that GM uses in their auto-trans. I figured that if GM will deny warranty claims for Max ATF in their auto transmissions, they're not likely to honor a claim using it in their manual ones either.

Cross-reference chart (PDF)

Synchromax Data Sheet (PDF)

Synchromax is recommended for manual transmissions that specify an automatic transmission fluid.
...
Synchromax is formulated with Royal Purple’s synthetic,
proprietary Synerlec additive technology, offering
improved shift quality over a wide temperature range and
reduced gear noise.
...
Viscosity Index 175

Max ATF Data Sheet (PDF)

Please note: Max ATF is not recommended or an approved warranty replace-
ment for: For dType F, Ford Mercon SP,Ford Mercon LV, GM Dexron VI,
Mercedes Benz NA G-2, Toyota WS, Volvo 5 Speeed.
...
Viscosity Index: 210

You'll note that, although both have exceptionally high viscosity index, the Max ATF is higher; meaning that in general it will provide favorable viscosity over a wider range of temperatures. However, the manual transmissions must cope with both reducing friction to minimize wear on gears while at the same time providing enough friction for synchronizers (synchros) to lock-up so your shifts don't sound "like a bag of marbles" or worse, wear down your gears. This is why high quality manual transmission fluids have both viscosity and friction modifiers or additives. I'm not a chemist so I can't tell you exactly how each works but you need both in the M6.

Do some Googling to learn more about manual vs auto transmissions and lubricants. That's what I did while researching this change. You will learn a lot more if you do the research yourself too.

Let us know how your choice turns out. Good luck!

--Cal