Rear diff, which lube to use?

[daffy]

I have a stock '00 X EB (110k miles), and was going to be changing the rear diff lube. When I picked up the stuff (Mobil 1 75W/140 +friction additive) I thought I had a LS diff(I had forgot to check before I left for the store). Anyways, I get home, and find I have axle code 42, which should be a 4.10 standard diff. Reading the owners manual, I find conflicting info. On page 266 it says:

"Vehicles equipped with 4.0L OHV V6 and 4.0L SOHC V6 engines and 3.73:1 or 4.10:1 rear axle ratios or with a 5.0L V8 engine require synthetic rear axle lubricant."

But, on page 268 it says:

4.0L vehicles with conventional rear axles are filled with Motorcraft SAE 80W-90 Premium Rear Axle Lubricant, part number XY-80W90-QL..."

OK, I have a 4.0L SOHC, and (I think) a conventional rear axle, with a ratio of 4.10. Since all the garages are closed, I can't call them, and was hoping I could get this done tonight.

Are there known problems with putting 75W-140 in a conventional alxe? If it won't make much of a difference, I'd just as soon go with the 80W-90 as it's nearly half the cost. Am I wrong to assume synthetic is better?

Thanks all!

 

[Blee1099]

I'd recommend 80W90 for a conventional axle (non Limited Slip) and 75W140 synthetic fluid with friction modifier for the Limited slip differential. 5.7 pints or about 2.75 quarts for the rear axle. I wouldn't recommend running the 75w140 gear oil in your conventional axle, I suspect the viscosity of the fluid would be too thick.

 

[high_horse_ford]

The stock lube is 75/140 synthetic and would last the life of the rear end. You would only have to change it if leaked out or got water in it. I installed an auburn posi in mine and drained out the synthetic and auburn says not to run synthetic so in went 80W90 with ford friction modifier no problems.