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PSA - Best oil for M5OD transmission

masospaghetti

Elite Explorer
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Year, Model & Trim Level
98 XLT, OHV, 4D, 4x4, 5M
The shifting from my M5OD (M5R1) is notchy when cold, and I've been trying a bunch of different fluids to see which are best. Wanted to share my results in case others have the same issue.

I've had 2 Explorers that collectively had 3 of these transmissions. The shift action of each was significantly different - For example, the original trans in my 98 shifted great (really smooth). The replacement trans in my 98, even with new synchros, hubs, and sliders, is notchy when cold. The transmission in my 2000 was somewhere in between.

Keep in mind this is all subjective. But, hopefully it will save someone some money before going out and dropping $50 on some fancy synthetic ATF.

- Mercon / Dexron: This is the factory fill, and one of the best for shift quality. Recommended.
- GM Transfer case fluid: As far as I can tell, this is exactly the same oil as Mercon / Dexron. It was repackaged specifically for transmissions and transfer cases when the newer Dexron VI / Mercon V fluids came out. More expensive than Mercon / Dexron, so not the best value.
- Mercon V: Despite Ford saying it is backwards compatible with Mercon, the shift quality was significantly worse (more notchy, brittle feeling). Not recommended.
- Mobil Synthetic ATF: Similar shift quality to Mercon V. Not recommended.
- GM Synchromesh: Similar shift quality to Mercon V. Not recommended.
- Redline MTL + Redline Lightweight ATF: This was recommended on the "Bob is the oil Guy" forum to blend a fluid with similar viscosity to regular ATF, but with the improved formulation of a true manual transmission oil. However, shift quality was very poor with this oil. Not recommended.
- STP ATF Type F: I put this in the transmission this morning to replace the Redline blend. Significantly better than the Redline blend, comparable with regular Mercon, maybe even better than Mercon. Recommended.

Note that this is only an evaluation for shift quality, not for wear protection or what's "best" for your transmission.

In short, I would stick with conventional Mercon or Type F fluid. Note that Type F is similar to regular Mercon without some of the friction modifiers. Supposedly, manual transmissions don't like friction modifiers (the modifiers were made for automatics). So why did Ford specify Mercon and not Type F? Best guess, Mercon was Ford's standard fluid at the time the M5OD was developed. Type F was essentially obsolete at this point in time, since the last transmission built that specified Type F was in 1981. But your guess is as good as mine.

Hope this helps someone and feel free to chime in your experiences.
 




RangerX

Elite Ranger
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Nice experimentation in the name of science! ;) That’s a lot of draining and filling!
I always assumed any not notchiness was the result of slave going out, or, subpar bleeding after slave replacement.
It never occurred to me that fluid could have an effect.
Good info and food for thought.
 




masospaghetti

Elite Explorer
Joined
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City, State
Huntington Beach, CA
Year, Model & Trim Level
98 XLT, OHV, 4D, 4x4, 5M
Nice experimentation in the name of science! ;) That’s a lot of draining and filling!
I always assumed any not notchiness was the result of slave going out, or, subpar bleeding after slave replacement.
It never occurred to me that fluid could have an effect.
Good info and food for thought.

For sure it's a good idea to check the slave and make sure the clutch has proper engagement.

Speaking of which, you got my wondering...so I had to check mine out.

If you have a helper it's fairly easy to check by pulling the rubber inspection plug on the side of the transmission bellhousing. I didn't have a helper, so I used my endoscope - this thing is super handy!

Below you can see the view thru that inspection plug - straight shot to the slave, throwout bearing and pressure plate.

1592155514342.png
 




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