I just read an article that says Tremec recommends ONLY Dextron3 ATF in the Magnum.
I had Amzoil in my wide ratio magnum and it shifted fine, my new Close ratio Magnum will have ATF so we'll see how that goes.
Decided to start a new thread dedicated to this discussion on fluids for the Tremec to keep the oil pan thread from drifting.agnum (and I didn't realize my shot of the bent pan above had the magnum), I had the following experiences with fluid:
Mobil1 ATF - D3 fluid - Way too thin when its hot, @BillyJRacing (Billy Johnson) recommended it as it was what was ran in the IMSA cars
With a cooler its better, but still warmer races I felt that it wore down the synchronizer rings more quickly than other options
Currently in the magnum is the BG (from OPM of course):
BG Products Multi-Gear Concentrate - MGC
BG Products Syncro Shift II Synthetic Gear Oil 75W80 API GL4 (which isn't a D3 fluid) - " BG Syncro Shift® II may be effectively used in all domestic and foreign vehicles where engine oil, ATF or GL-4 gear oil is specified as “service fill” for manual transmissions. Also suitable for use where GM PN 12345349 (specification 9985648) FORD PN D80Z19C547A, PN ESRY19C547A and MERCEDES-BENZ PN 900260315 are specified. "
This fluid works much better for me, slight increase in shift effort when its cold, but usually by the end of the warmup lap that's over and done with. Much better protection and shifting characteristics it seems even when hot, especially when the idiot driver forgets to turn on the transmission cooler (plumbing an automated circuit for that soon). My magnumXL is setup with a number of brass and carbon parts, not many organic lining parts in it so that may impact fluid selections for some. (I believe most magnums are that way but a TR6060 might be pickier about fluids)
Quick backstory: Converted to the Magnum XL in December 2019 after the MT-82 gave up. The conversion kit I used from Modern Driveline included the Tremec HP-MTF, which is now their spec fluid for the Magnums. Ran the HP-MTF for 8 track days from January-May, in which time the transmission performed really well. A bit notchy when cold, but seemed to be very much in line with everything I had read about the Magnums and never had any gear clash whatsoever.
Decided to change the transmission fluid early June, at which time the HP-MTF wasn't in stock anywhere. I run Royal Purple throughout the car, so decided to make the switch and give the Synchromax a try. I've since read that Tremec's can be finicky with fluids, but would never imagine how drastic the issue can be. On that first track day, it started to have gear clash on 4-3 downshifts. What's strange is it's only when cold, then by the end of warm up lap it downshifts fine. Still not believing that a fluid change could cause this, I bled the clutch, verified the clutch was disengaging properly, etc., which all checked out OK. Ran one more day at Buttonwillow, and exact same thing - gear clash when cold - good to go once warmed up.
Switched to Mobil 1 ATF, ran a few heat cycles (same gear clash), drained and refilled again with Mobil 1 ATF. Still the same. Tremec finally had the HP-MTF back in stock, but it wouldn't arrive before the next track day. Switched to TORCO MTF, based on the viscosities being the closest I could find to the Tremec HP-MTF. Marginally better, but still clashes when cold. Now that I have the HP-MTF here, I plan to change back before Auto Club 2-day on Oct 3&4. As you can imagine, my confidence isn't very high that this will magically remedy the problem. I'm concerned that the fluid change actually damaged the 3rd gear synchro, and I fear that I'll have a fight on my hands trying to pursue a warranty claim with Tremec since the RP Synchromax is not one of their "approved" fluids. Again, hard for me to imagine that a fluid can (or should) have this much impact. Bowler Transmissions spec'd the Royal Purple Synchromax for their Magnum kits. Maybe if you start with that and stay with it, everything is fine? Beyond frustrating having this type of issue when the whole rationale of dropping $6K on the conversion was to eliminate the worries about the transmission.....
The chart below is the cold and hot viscosities of several different ATF's and MTF's for reference