How much gear oil will I need when changing the front and rear gears? Do I need to buy Royal Purple or can I buy another synthetic oil and add the friction additive to the rear?

Thanks in advance

 

what kind of truck do you have? My owners manual for 1997 F150/250 says front axle 4wd 1.65L (3.5 pts). Rear axle F150 2.6L (5.5 pts.) Rear axle F250 3.3L (7.0 pts.) For F150 Ford 22.35 cm (8.8 in) and 24.76 cm (9.75 in) Ring Gear Conventional and traction-Lok add 118 ml (4 oz) of additive friction modifier. For F250 add 8 oz. for complete fill.

 

There should be a tag on the differential cover that will tell you what weight and viscosity to use, the only time you need to use a friction modifier is on a limited slip rear axle, in that case the tag would say a synthetic 75w-90 or 75w-140. in that case 8oz. of friction modifier is necessary.

 

the old 80-90 wt gear lubes have been replaced in newer trucks with 75 w 140. Older trucks will have the older tags,. 75-140 is the new spec. Synthetic oils last longer and dont loose addatives, and suppose to give you a few % better gas milage due to less drag. I have bought the generic synthetic gear lube at Wal Mart, and it meets all the specs. If you want roay purple or other and pay the extra $$$ thats your choice, your not getting much different from the generic. If you truck has a limit slip, you will need to add the friction modifier. Auto Zone sells a 7 oz tube for $10. just squirt it in then add the lube. I use the whole tube on a 9.75 axle- works great.

 

My manual says for an 8.8 or 9.75, the rear gear oil is considered good for lifetime. What is considered lifetime?

 

THe oil addatives will last forever, but the sand in the casting, wear on the gear faces, chips left inside... wil all grind up and go thru the bearings. Do yourself a favor and replace it once--- then it can be life time ! I have bought several vehicles at 20k miles after they are a year old. First thing i do it change ALL the oil.. not just motor. Do the axle (F and R), tranmission, transfer case, check the brake fluid. YOur truck will thank you 10 years later.

 

My 99 says a lifete along with my 05 and after 130k they still look good. If there is a leak or if the axle ever gets fully submerged in water, it needs to be changed. As for the friction modifier, most synthetic gear lubes have ls additive in them and they will say so if they do. Put the new fluid in and drive it and make some tight turns and see if there is bucking and it locking up. If so, add about half the modifier and test again. Add modifier until it's smooth when turning sharp. U don't want too much additive or it will become a limited grip

 

dont look at whats floating on the top at the dip stick or fill hole. Pull the pan and look at all the goooo in the bottom.

 

So, go with the off the shelf synthetic 75w140 for limited slip axles? I'll pull the cover and drain it and change it. Do you prefer regular gaskets or just rtv black? I just used rtv for my jeep differential covers.
Also, does the front axle have to be synthetic as well? Thanks

 

Generic synthetic at Wal Mart or Auto Zone is fine. I have not bought a pan gasket in 20 years. New transmission have reusable rubber, and axles are RTV. I prefer the orange RTV, but most are fine. Most front axlels still spec 80-90 straight gear lube if you want to save a buck.

 

Yes, my 99 f250 takes regular 75w-90 in the front

 

I've got some 80w90 Valvoline conventional left over from my jeep. That'll work for the front, won't it?

 

That should be fine. There isn't much of a difference between 75w and 85w. Besides, the front end doesn't get used much so it isn't as important as the synthetic put in the rear

 

Pumped in the rear nearly 3 quarts with only some residual left in the bottles, and it isn't flowing out of the fill hole yet. I can get some on my finger sticking it in the hole. I wouldn't think I have 0.3 pints left in the bottles. Does the specified 5.7 pints actually level out below the fill plug?

 

no, you fill it till it runs out the hole. level with the threads.