Gear Oil

[Melon]

I read somewhere a while ago about a TSB and using 75w140 for the rear? Is this legit and needed? Or can I use 75w90 for front and rear? Plus adding a bottle of friction modifier in the rear differential?

I think the R&P is howling in our 2004 so I'm going to see if a fluid change will help it. I may even splurge and go with Amsoil for it. Plus it has 130k on it anyway so changing the fluid probably wouldn't hurt.

Thank you for the help

 

[TechGuru]

Using thinner oil certainly won't do anything to help the noise, if anything you'd want to go thicker.

75W-140 is spec for the rear.

I use:

 

[ROBinGa]

Only add the friction modifier if you rear axle calls for it. You should find an axle code printed on sticker in your driver door and that is if you are sure it’s the axle that came with your explorer stock. My 2003 XLT 4x4 with 4.6 has a rear axle that calls for NO friction modifier. I have replaced the rear diff gear oil using 75w140

 

[TechGuru]

Friction modifier is for the limited slip differentials.

 

[kydirtscooter]

On my 03 which has the 3.73 limited slip, I ended up using 75/140 in the rear and no friction modifier. Read the 75/140 bottle carefully as some come with modifier already in it. If you are an open differential, it is fine to use the gear oil with modifier already added it just won't make any difference in that case.

I have noticed a slight whine for a long time at 50-55mph and it is reduced after changing the gear oil. My magnet was pretty clean (fill plug).

 

[Melon]

Thanks everyone. I'll get 2 quarts of each weight and service both differentials.

I know friction modifier only gets added if I have a Limited Slip. I should have added that part in my original post

 

[firefly1246]

FYI: if you have limited slip (comes with tow package) make sure even if the gear lube says it has friction modifier already in it you MUST add a bottle of friction modifier. Your diff will howl around corners if you don’t. I can tell you from experience. I put Royal Purple in my rear diff which states it has friction modifier but clearly doesn’t have enough.

 

[Melon]

Thanks I had planned on adding a bottle to be on the safe side regardless as to what the oil bottle said. Atleast then I would know I had the appropriate amount instead of hoping and guessing.

 

[BKennedy]

Rear diff takes 2.8 quarts, so get 3 of the 75-140. I started using Lucas because its recommended by most of the aftermarket locker brands. Found it on Summit Racing cheapest a few months ago

 

[TechGuru]

Rear diff takes 2.8 quarts, so get 3 of the 75-140. I started using Lucas because its recommended by most of the aftermarket locker brands. Found it on Summit Racing cheapest a few months ago

Not quite sure where you got that information...

It's 1.625 quarts...

OReilly's sells the Motorcraft Friction Modifier:

https://www.oreillyauto.com/detail/...1efcf6a5e17d/motorcraft-additive/xl3/4493781/

 

[CDW6212R]

Not quite sure where you got that information...

It's 1.625 quarts...

OReilly's sells the Motorcraft Friction Modifier:

https://www.oreillyauto.com/detail/...1efcf6a5e17d/motorcraft-additive/xl3/4493781/

Interesting, the later diff's are smaller than older 8.8's in volume. It typically takes over two quarts to fill an 8.8 rear, ending up with 3/4 of a bottle out of three you start with.

The Amsoil is $12-14(75W-90, 75W-110, 75W-140) a quart now, it went up about a dollar last year. For a $15 membership cost, you can buy anything they have for the good prices. Their synthetic lowest level engine oil used to be $5 a quart, I was using that for years until last Fall. But they have tons of good products, and you only want to consider them if you have the membership for the prices.

Look at the door jamb VIN tag. Find the axle code, if it's a letter, the rear diff is an LS and needs the friction modifier. If it's just a number, you don't need it.