Jaime,
Here's the short answer about which fluid is correct from a friend of mine who operates Pro-Force Performance.
World Class - use Dexron III, Mercon-V, or Mobil-1 Synthetic DEX/Merc ATF.
Non-World Class - use Synchromesh
This is a long answer, but a pretty thorough one. The internet will give you a variety of answers about which fluid is correct for a T5. Most of the fluid
advice I have read on the internet comes from people who write stuff like "I've been using such and such fluid in my T5 for 2 years now and never had a problem".
To give you the most accurate answer possible, I referenced an actual T5 service manual. This is a quote directly from the T5 service manual. (Complete with the typographical error!)
2-4. APPROVED LUBRICANT. Most T5
transmission models use Dexron II automatic
transmission fluid. Refer the vehicle owner’s manual
or service manual for lubricant specifications.
CAUTION
Do not mix different bands or types of transmission
lubricant. DO NOT USE GEAR OIL IN THE T5
TRANSMISSION SINCE THIS MAY DAMAGE THE
BLOCKING RING MATERIAL. My friend Stan runs
Pro-Force Performance in Atlanta, GA. He specializes in performance Mustang manual transmissions including T5 upgrades and rebuilds. He explained to me that GEAR OIL will eventually degrade the fiber friction material used in WORLD CLASS synchros, and thus ruin them after time. That is the reason for the warning in the T5 manual I quoted above.
Your T5 is a non-World Class. It has brass synchros that don't have a fiber material, so the warning about gear oil really doesn't apply to your T5. Still, what fluid did your T5 come with from the factory? I wanna know.
The T5 service manual doesn't specify what fluid to use in a non-world class T5, but refers you to the vehicle owner's manual. So I went online to the
GM Heritage Center website. It has really cool original info to download about all sorts of older GM vehicles.
This internet chart indicates your 13-52-145 T5 came from a 1985 or 1986 S10 style truck. The GM Heritage info for the 1985 and 1986 S10 truck doesn't list info about the T5 fluid. Therefore, I consulted the GM Heritage Center for the 1985 Camaro info.
Pages 66 and 67 of the downloaded 1985 Camaro info say to use Dexron II in the 5 speed. (Pages are labeled 8A and 8B at the bottom).
The internal parts of an S10 and Camaro Non-World Class T5 are the same except for gear ratios. So my conclusion is that Dexron II ATF is the original fluid for the 1985 S10 as well.
So I suspect that GM may have used gear oil inside the 1983 or 1984 T5 models and that's why the T5 service manual even bothers to mention gear oil. I think the Mustang T5 was WC from 1985 and up. GM started using WC in F body cars 1988 and up and S10 trucks 1993 and up. So it makes sense that any T5 service manual published after 1985 would have the gear oil warning.
Dexron II is no longer made. Dexron III ATF is thinner than Dexron II. My friend Stan at Pro-Force Performance says that Tremec (the company that bought the rights to the Borg Warner T5) now recommends Synchromesh for Non-World Class. For the World Class T5 use Dexron III, Mercon-V, or Mobil-1 Synthetic DEX/Merc ATF.
I have no idea why your transmission man thinks gear oil is best for a hot dry climate like NV. The Non-World Class people in the hot rod community have experimented with different fluids since they don't have to worry about ruining the fiber material on the synchros. Some say that ATF makes for smoother shifting. If you use gear oil, I really can't tell you what weight to use.
I plan to use Synchromesh in my NWC T5. I certainly can't disagree with using Dexron III ATF or similar.
