Tranny Fluid

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Nov 14, 2025 | 07:46 AM

My93F250NH

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Mountain Pass

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Tranny Fluid

1995 F250HD, ZF5, 460, 113k miles

My transmission shifts and drives, but you feel it "touch" the gears on most shifts. Not a complete grind, but you know that two gears have come together.

I don't know what fluid is in the transmission, so I'd like to change that out in an effort to perhaps make the shifts a bit better. I suspect the synchros are near end of life, maybe I can squeeze out a little bit more life. I only plan to put 1k to 2k mi per year on the truck.

Any recommendations for transmission fluid for an older ZF5? Something to help the synchros do their job a bit better? I know Mercon is the requirement, just wondering if anyone has had good luck with a particular brand, or synthetic, for this specific use case (older tranny, prob needs synchros).

Last edited by My93F250NH; Nov 14, 2025 at 07:55 AM.

Nov 14, 2025 | 12:02 PM

knottyrope

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From: Bahstun

Mercon V Synthetic is what i use in mine

Can you shift into first easy? If not the clutch might need to be bled. I take off the slave and compress it ten times fully. Also drain the fluid by dipping in a rag and adding in some fresh fluid helped a lot.

Nov 14, 2025 | 01:06 PM

My93F250NH

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Mountain Pass

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Quote:

Originally Posted by knottyrope

Yah, first seems to be okay. Most noticeable problem is third. Doing a bunch of research this morning, I ordered Redline MTL. I'll update the thread and let you know if it made any difference.

Nov 15, 2025 | 09:44 AM

1Butcher

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I use GM Synchromesh, sure, it's for a VW transmission, but it helped my shifts.

If the clutch is dragging, that could cause your issues too.

Nov 15, 2025 | 09:51 AM

My93F250NH

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Mountain Pass

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Quote:

Originally Posted by 1Butcher

I use GM Synchromesh, sure, it's for a VW transmission, but it helped my shifts.

If the clutch is dragging, that could cause your issues too.

Clutch is new (along with throw out bearing, flywheel etc.). But who knows. It's entirely drivable, just like to hunt down potential issues. I'll report back later today (hopefully) on the Redline.

Nov 15, 2025 | 10:38 AM

My4Fordtrucks

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Quote:

Originally Posted by 1Butcher

If the clutch is dragging, that could cause your issues too.

I agree. Since this:

Quote:

Originally Posted by My93F250NH

My transmission shifts and drives, but you feel it "touch" the gears on most shifts. Not a complete grind, but you know that two gears have come together.

seems to be all the gear changes with 3rd being the worst. That is usually a clutch issue.

Quote:

Originally Posted by My93F250NH

Clutch is new (along with throw out bearing, flywheel etc.). But who knows. It's entirely drivable, just like to hunt down potential issues.

That doesn’t mean anything if the hydraulic system isn’t fully releasing the cutch disk when shifting. Do you have a good, firm pedal? Is the firewall flexing when the pedal is pressed? Are the bushing on the pedal assembly still there and not broken or missing?

Nov 15, 2025 | 11:14 AM

My93F250NH

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Mountain Pass

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Quote:

Originally Posted by My4Fordtrucks

I agree. Since this:
That doesn’t mean anything if the hydraulic system isn’t fully releasing the cutch disk when shifting. Do you have a good, firm pedal? Is the firewall flexing when the pedal is pressed? Are the bushing on the pedal assembly still there and not broken or missing?

Thanks for the insight. I replaced the clutch pedal shaft bushing. I know it's temporary, but ok for now. I didn't see any ovaling of the hole. The old bushing was there, but near the end. Pedal feels very firm. I haven't checked for firewall flex but will. I'll get the fluid changed and let you know where I am.

Thanks again for the help.