75w140 or 80w90?

Mike Tonon · Apr 13, 2015

07 Ranger 4x4 with rear limited slip (yes, I put friction modifier in), 8.8, 4.10 differentials

I replaced my gear oil yesterday with 75w140 synthetic from the dealer, because they said the 75w140 supersedes it. Is this ok or am I going to destroy bearings? (But they also told me Mercon V was recommended for the transfer case, when it says right on the back not to use it in transfer cases.)

My area has winters that typically get down as low as -5 F or so, a few nights.

My repair manual calls for 80w90 for front and rear, but I've read that newer 80w90 is different than it was then.

What's with all this confusion on correct fluids????

4x4junkie · Apr 15, 2015

You won't destroy bearings by using 75W-140 in place of 80W-90 in a differential.

What you may lose though is a very slight bit of MPG, since it is a thicker viscosity than what you need (probably not more than 0.5-1 MPG).

I also don't think Mercon V will harm a transfer case (at least not a 1350 or 1354), however I would agree about using what is recommended, which nowadays is known as "Dex/Merc" fluid, since Ford decided to go stupid with licensing of the name "Mercon" some years back.
Indeed that has caused a lot of confusion when it comes to fluids for pre-'08 Fords where Mercon fluid may have been called for (Ford flip-flopped a couple times on whether Mercon V was actually backward-compatible with Mercon, eventually settling on a claim that it superseded Mercon, however it's been observed to cause degradation of shift quality on some M5OD transmissions, so I do not recommend it's use in the M5OD).

gw33gp · Apr 15, 2015

Ford specs synthetic 75W-140 only for the FX4 LII differential with the Torsen. The bearings work just fine with it.

adsm08 · Apr 15, 2015

gw33gp said:
Ford specs synthetic 75W-140 only for the FX4 LII differential with the Torsen. The bearings work just fine with it.

Ford specs 75-140 for most things after 2010. The other thing with that is that they often issue supercessions of fluid specs in the form of Special Service Messages or Technical Service Bulletins, or worse yet as a footnote or update in the parts catalog, and then never go back and update the shop manuals.

As I have pointed out before, even once in a thread started by Mr. Tonon, Ford says to use Transfer Case fluid in the transfer case. The only difference between T-case fluid and Merc V is T-case fluid has limited slip friction modifier in it to help with engagement of the gears and operation of the magnetic spin-up clutch. And about $6 on the price of a quart.

I have used Merc V or equivalent. in many transfer cases, including my own, to no ill effect.

Mike Tonon · Apr 16, 2015

4x4junkie said:
it's been observed to cause degradation of shift quality on some M5OD transmissions, so I do not recommend it's use in the M5OD).

What do you recommend? And what's different about Mercon compared to Mercon V?

4x4junkie · Apr 16, 2015

Mike Tonon said:
What do you recommend? And what's different about Mercon compared to Mercon V?

"Dex/Merc". Another variation might be "Multi-Vehicle ATF".
Valvoline, Castrol, and others still make it, though you won't find it under the Motorcraft label or at a Ford dealer anymore.
The label will say: "Meets Dextron III and Mercon requirements" or something similar (along with a note: "Do not use in applications calling for Mercon V").

Mercon V is a semi-synthetic fluid. My best theory is that it contains an additive that is too slippery for the synchros in a manual transmission to function as intended, making for harder, and/or clunkier shifting (including difficulty getting it into gear while stopped, which can be easily mistaken for a clutch issue).

I recall there were a few guys awhile back who also used 0W-30 synthetic engine oil in their M5ODs, with good results, though I haven't seen much discussion about it lately, so I can't say if this is still common or not.

gw33gp · Apr 17, 2015

I have been using Royal Purple Synchromax. It provides improved shifting compared to the original fluid Ford put in it. I have over 180K miles on the transmission and most of it has been with Synchromax. The only problem I had with it was a small leak at the housing/extension interface due to poor application of sealant by the OEM. Otherwise, it still works like new. It does seem to degrade over time and needs to be changed around 60K miles.

I use Mobile 1 ATF in my transfer case.

75w140 or 80w90?

Mike Tonon

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adsm08

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Mike Tonon

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