2005 Ford Ranger FX4 Off-Road

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This is the "neighbor's truck" I purchased in August which is still under 50K.
Being 16 MY old I want to change the differential fluids.
80W-90 is the specced fluid. I want to go synthetic as they're uniformly suggested. Also, the FM for the rear is already in the synthetics.

A few people urged me to use a 75W-140. This is the lube for the Level II's Torsen rear.

How does one calculate any benefit. With my truck's low mileage I'm thinking the specced 80W-90 is fine.

Any towing will be rare and done in temperate climates.

I asked a similar question a while back but deciding between conventional lube and synthetics was the focus then.

I am going synth. Is the 140 even to be considered?
 
I purchased my 2008 Ranger 4WD XLT @ 74k with the regular 3.73 rear differential maybe two months prior to your original post.

I used the 75W140 since like you, do not use the truck in the winter. I chose to go with the 75W140 since I will tow a bass boat and fill up the bed with lumber on occasion.

Truck did not seem to mind while running it up the Northway after the change.

Will be changing out the transmission and front differential fluids as the Spring gets underway to complete the base line history on the fluids.

Looks like Rand hit your mark.
 
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At one point Ford issued a TSB specifying 75W140 in place of 80W90 in some limited slip 8.8" applications (non-Torsen) to reduce "chatter" and other noises.

I used it in the 7.5 in my old 2.5L I4 Ranger because I figured what's good for an 8.8 must be good for a 7.5. It worked fine and had no noticeable impact on fuel economy. In fact the best MPG I ever got in that truck was with the 75W140 in it.

The only synthetic 80W90 I know of is Amsoil. 75W90 and 75W140 synthetics are what you will find in stores.
 
According to my Amsoil rep, 75w140 is pretty much perfect for anything calling for 75w90 or 80w90.

You'll do fine with the 140.
 
I'm pretty sure Ford has re-spec'd most rear 8.8 diffs to 75w-140. There is really no good reason not to unless your some sort of fractional gas mileage freak. A lower winter rating and vastly better protection for hard use (off road, towing, aggressive driving etc).
 
Ford back spec'd all rear axles to synthetic 75w-140 around 2005. I purchased my 2005 Ranger new and after 6 months they rebuilt the rear axle under warranty. They put in 75w-140. Since then I have changed the rear axle oil with SuperTech syn 75w-140. I don't know if the specs changed for the front axle.
 
I was never aware that they were back spec'd. I have a 04 Sport Trac, which basically has the same setup. But I don't tow or haul heavy either. I just replaced them with 75w90, but if I knew they were backspaced, I would of pushed for 70w140. Like 01rangerxl said, you will be hard pressed to to find a synthetic 80w90, so I really don't see a problem using 70w90 or 70w140 instead. It would be nice just to keep both axles the same, that what I would do.
 
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