I just changed the transmission fluid in my TKX, which takes ATF, and I used the same pump I used to use for the gear oil in the old Muncie GL4 or GL5. I sent some brake cleaner though it and put a little ATF though it first that I discarded. I’m sure it doesn’t matter since it’s all oil and it’ll change the viscosity by 0.001% and there’s no clutches anyway since it’s manual.
But it got me thinking about all the posts I’ve seen about how gl4 or gl5 will eat your bushings and synchros and all that and you must use the correct one or you’ll grenade everything. Does anyone have any actual experience on this or know because the information I’m getting is conflicting. I’m not really concerned about it more just curious and it’ll be helpful for when I eventually need to change the diff oil.
Thanks!
I’d be more concerned about the brake cleaner you used to flush the pump with than the gear oil.
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At 1st I thought you had swapped over to GL4-5 from ATF… lol
Cross contamination between fluids on that small of a level (if any) shouldn’t hurt anything after cleaning the pump and then running some old ATF through it… I am sure it happens a lot in small shops… I personally have not seen or heard of any long term issues from what you have described, but I am more into the Automatic side of the game…
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You cleaned it better than I would have so I guess you know my opinion on it…
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And you’d be wrong on that point. Oils are designed for specific tasks, they are not universal.
ATF is designed so that clutches are lubricated but don’t slip when engaged. GL4 or GL5 are designed to be as slippery as they can be to reduce sliding friction between the ring and pinion. The synchos are designed for the fluid required by the manufacturer. Using GL4 in a trans designed for ATF will not make your synchros happy and may destroy the bearings if they were not clearanced for the FAR thicker GL4 or 5.
I’ve run 5W30 Mobil 1 in an 84 Corvette (basically a T-10 4 speed) and it worked great. It is far more similar to GL4 than it is to ATF so the synchros and the bearings were happy. Everything after that car used ATF, including a FWD car that bathed the ring and pinion with transmission fluid.
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Sorry that was a bad way to put it, I know they have different specs I just meant I’m not throwing water or kerosene in there but more of similar liquid haha.
That’s interesting that the T-10 was happy with an oil that light. I was under the impression that it was sort of similar to the Muncie and all older transmissions needed heavy gear oil.
It freaked me out when I first got my tkx and I remember reading the manual 20 times over to make sure it actually wanted atf. Didn’t know it’s a common thing now!
I left it for a bit over a day after the brake cleaner if that makes it any better. Couldn’t smell it so I think I’m hoping it evaporated away.
They all required gear oil for sure. When I bought the Corvette used, two engineers from Cadillac suggested I try 5W30 synthetic. They got the information from a Powertrain test that showed it shifted better, improved mpgs a bit and had no effect on wear. I was a GM engineer at that time as well. It did shift much smoother than with gear oil.
Your TKX trans is a better T-5 transmission. The T-5 always used ATF. The T-5’s limitation was never about oil, it was about strength! A t-5 is marginal for a 5 liter V8 even in a light car. I never had bearing trouble with my T-5 but I did break more than a few 3rd gears and input shafts!
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Most GM manual transmissions now call for ATF Dexron III or IV. My 86 GMC S-15 pickup called for Dexron II. Friends 2017 Vette with 7-speed manual calls for Dexron III.
My 98 pathfinder had a service bulletin on using GL-5. Said to stick with GL-4 because GL-5 could harm brass components in the transmission.
And I bet you had a lot of fun doing it!
After I was inducted into the broken hub club (2x) this guy says to me- dude, it’s only metal 
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Yes, I did! I had a spare for my race car that we carried with us. I broke one at a race and my crew chief and I had it swapped in about 25-30 minutes. No wires or speedo cable to worry about made it quicker. Left plenty of time for lunch before the next practice session.
I got very good at rebuilding T5s. When I got fed up with the T5, I bought a Richmond T-10 4 speed. If your arm doesn’t hurt changing the gears, you aren’t shifting hard enough!
Seems like not much to worry about. But since it concerns you, maybe you can come up with another way to pour the fluid into the trans for the next time. Instead of the pump, maybe a short plastic squeeze bottle, refill as many times as necessary. That’s how I fill my truck’s rear differential. There may be another place to fill as well, like the transmission vent port.
Best I’ve been able to find is that early GL-5 was hard on brass in the gear box. I hear that most producers have solved the incompatibility. I wouldn’t hesitate to use GL-5 if it’s recommended by the OEM, but I would not override a spec for GL-4.
On the contamination issue, I’ll usually flush any hoses or equipment with whatever it is I’ll be using before actually filling an assembly. I do what I can to minimize cross contaminating fluids, but I see no reason to obsess over it.