I'm in the process of rebuilding my '89 F150 and drained all the fluids from the tranny and transfer case.

I have a Haynes manual that calls for Mercron ATF for the M5OD. I'm confused as to what they would spec out a thin fluid like this for a manual tranny. I had planned on using gear oil but after I RTFM I'm not sure what to use.

What are the negatives if I went ahead and used the gear oil to refill the tranny?

Thanks

 

My 92 Explorer uses that in the transfer case and the tranny.

 

I never have understood putting such thin oil in a MT either. But i have read that some bearings are splash fed & if you use gear oil it wont splash. To be sure of what fluid it takes look in your owners manual. It will show the Ford spec, then just do a google search on that spec.

 

Exactly, with the gear oil the splash-fed bearings don't get their proper lubrication. Too thick. The ATF works just fine though, it has for me so far since I made the switch BACK to ATF from gear lube. I had a 3rd gear grind that the ATF has all but solved, and it shifts much easier now.

Another problem with the gear oil is that it wants to chew on the synchronizers. This requires the use of a special gear lube, one that is friendly to "yellow" metals - you have brass synchro's last I checked. GM has a GL4-spec gear oil that fills this...and the other that I know of is the one I recomend. Redline makes a fluid (MTL I believe) that has a good pour characteristic, being a MTF, will splash, and offers the smoothness and best of all worlds. Plus, its a synthetic product so you have that added 'relief' factor.

For the life of me..I forgot his name again. An individual here on the board is also using a Honda manual transmission fluid...racing bred, shock modifiers, all those good things. I need to write him down somewhere. Sorry!

 

Like saurian sez, if you want to get maximum miles out of your trans, use a good synthetic like Redline or in my case-87 Ranger 5 speed which does use gear oil-i use Amsoil 80-90. I think the reason that my T-K is stll going strong at 223K is thanks to less friction with the synthetic.

 

I was under the impression that gearboxes that used ATF were pressure fed from an internal pump. Gear oil being too viscous to pass through the screen fast enough to prevent the pump from cavitating and starving the bearings of oil. The BW transfer case on my truck shows a pump and screened pickup in the exploded diagrams. Of course it takes ATF, so that's how I made that connection.

 

When I had my M5OD open, I saw none of that. even when it was stuck in first with the top cover off, when I let out on the clutch it started to spin up in the front and splash ATF up. I killed the rotation before it could really shoot, but it was all from the ATF riding up in the gears of the spiral cut gears.

 

Scott,

Yes, the transfer case has a pump, but the M5OD does not. It does have a "guide" to direct splash from the gears.

Jim

 

Synchronisers will not work with heavy oil. The rings have to be able to drag on the
tapers of the shafts to bring the shaft speeds together so a clean shift can be made.
The heavy oils don't let this happen, if you look up the requirements for the Clark
five speeds and others 80/90 was never called for it was 50 weight engine oil. I just
drained a Ranger MOD that didn't shift clean and was hard to put in gear cold. Once the heavy oil is gone they shift like a dream.

 

I once put 20w50 motor oil in my Mazda 5 speed and it would hardy shift.

 

Thanks for clearing up my confusion!

Scott

 

Redline MTL (manual transmission lubricant)

I have used this stuff in T5's, NP435's, and M5OD's

Always have seen a significant improvement over whatever was in there previously.

 

Even with cold starts in winter? I put Redline D4 ATF in my M5OD because the MTL is thicker and I was concerned about cold shifting, which already is a bit rough on my tranny.

 

Yes, especially in cold weather. The syncronizers job is to stop the gears from spinning while shifting so that the gears can engage without grinding. The MTL has a viscosity of 30wt motor oil, and has the proper coefficient of friction for rapid syncro engagement.
If your oil is too slippery, your syncro's can't slow down the gears fast enough for a proper shift. For example..... if you put a full synthetic motor oil like mobil one into a Ford T-5 it will NOT shift right...... and will lead you to think that the syncros are going out.

Changing to MTL from regular 30wt motor oil in an NP435 makes a noticeable improvement of shifting into the 3rd and 4th gears. However it makes no difference on shift quality of 1st and 2nd because only third and fourth have syncronizers.

So, to answer your question...... yeah it works great in cold weather in the M5OD.

 

Well I can't dump 4 qts of Redline just like that. It's only been in a few thousand miles. But I could use the D4 in the Aerostar automatic tranny. Would that be a good idea or is there stuff gotten into it from my manual tranny that would wreak havoc on the automatic tranny?