Hi,

This is my first post, so hello everybody, I'm Adam from Ruston, LA and I am a student at LaTech.

I got an '05 Navigator with 170 k miles on it. I am the second owner so I don't know if it had any dif. oil change before. So I thought its better to be safe than sorry.

I am planning to buy 3 qts Mobil 1 75W-90 for front axle and 2 qts 75W-140 as its specified in owner's manual. Anyway, my question is Do I need to add any friction modifier to it. It's 4x4. Mobil says it already has friction oil in it and no need to add more, but also says can be added if manufacturer recommends to.

Also, my regular repair shop requires $150 for labor of front and rear oil change. Is this OK? I see lots of videos for DIY but I'm afraid if something goes wrong I could be charged double.

Thank you and drive safe.

 

You only need friction modifier if you have a limited slip differential.

However, here is a footnote from your owner's manual:
Your vehicle’s rear axle is filled with a synthetic rear axle lubricant and is considered lubricated for life. These lubricants are not to be checked or changed unless a leak is suspected, service is required or the axle has been submerged in water. The axle lubricant should be changed any time the axle has been submerged in water.

 

Really? So you don't think I should change them even after 171k miles?

 

If those that designed the vehicle don't think you need to change the fluid, then who am I to argue with them. Of course the footnote specifically mentioned the rear differential and made no mention of the front one.

That might have been an oversight since some vehicles don't have a front differential, or it could mean the OEM fluid in the front differential isn't synthetic and should be changed.

But on a final note, since there was no mention of a mileage interval for changing either the front or rear differentials, then one could safely ***/u/me that both differentials are lubricated for life and do not have to be changed.