75W140 gear fluid, change?

Seraphitia102 · Jun 16, 2005

I went into EZ Lube and they said that I should get a rear gear fluid exchange and it wasn't too expensive so I did it, and they used 80W90. When I looked under the bottom of the car, it says "75W140 synthetic gear oil only" or something like that and I went back to EZ Lube and they replaced the oil for me (some 3 months later).

I read the manual of my 2001 Mustang GT Convertible and it says that the 75W140 fluid never needs to be changed for the life of the vehicle, that it stays permanently lubricated once locked into the oil case of the rear differentials. Is this true? Did I even need to have my rear gear fluid changed in the first place? Or should I just disregard what the manual says and keep changing the rear gear oil every 35,000 miles?

SigmaPride · Jun 16, 2005

Yeah. My '99 calls for 80w90 and also says that it never needs to be changed unless it is leaking or has been submerged in water. I think starting in '99 they stopped saying you needed to change it, but I thought 80w90 needed to be changed. Not sure here...

40th GT · Jun 16, 2005

Depending on your driving, the diff fluid gets dirty at around 25k-50k so I would call bs on what the manuals says. It says to keep original plugs in there for a 100k, lol they'll probably seize in there by then..j/k
I would however go by the manual and use 75w-140 w/ friction modifier because it says the same thing on the rear end cover. And you won't need friction modifier if you choose to buy synthetic fluid.

Seraphitia102 · Jun 17, 2005

40th GT said:
Depending on your driving, the diff fluid gets dirty at around 25k-50k so I would call bs on what the manuals says. It says to keep original plugs in there for a 100k, lol they'll probably seize in there by then..j/k
I would however go by the manual and use 75w-140 w/ friction modifier because it says the same thing on the rear end cover. And you won't need friction modifier if you choose to buy synthetic fluid.

Do you know how many quarts I need to buy to fill the differential fluid?

prsrizdgt · Jun 17, 2005

Seraphitia102 said:
Do you know how many quarts I need to buy to fill the differential fluid?

Look in the capacities section of your owners manual. It will tell you how much to put in. In reality, all you have to do is fill it until it starts coming out of the fill hole. You do not have to change the gear lube before the 100,000 mile mark, but as most of us car lovers are maintenance freaks, it certainly never hurts to change it sooner, and the unneccesary costs are out weighed by how much better you feel about doing it. Unlike engine oil that gets contaminated by fuel and other waste products in the engine, your rearend gears and bearings live because of basic lubrication. Gear lube will lubricate for many miles, hence the long service life.

Seraphitia102 · Jun 17, 2005

what happens in a scenario where let's say someone doesn't do the job right, would your rear end gears cease to function right away or overtime? i know it's a stupid question, but I was just wondering in the worst case scenario where the job was done incorrectly.

hotrodnut · Jun 17, 2005

The gears would not be effected by the different lube but over time the clutch plates in the limited slip could be. Once the plates are saturated with one type of oil, especially synthetic and you switch to mineral based oils and back to the synthetic oil the plates may not function as designed.

Dark Knight GT · Jun 17, 2005

Gear lube is rather inexpensive to change. If you can afford it, just change it once a year. Not that it's needed, but its cheap insurance. You're rear differential holds close to 2qts. If you have an aftermarket girdle, like I do, then it will hold a slight bit more.

Seraphitia102 · Jun 17, 2005

Dark Knight GT said:
Gear lube is rather inexpensive to change. If you can afford it, just change it once a year. Not that it's needed, but its cheap insurance. You're rear differential holds close to 2qts. If you have an aftermarket girdle, like I do, then it will hold a slight bit more.

Any brand that you recommend? Is there one fluid that's better than the other?

slo65 · Jun 17, 2005

used redline in mine, then again i have a fox

Ernad · Jun 17, 2005

slo65 said:
used redline in mine, then again i have a fox

Yeah, so wtf are you doing here?

Dark Knight GT · Jun 17, 2005

Seraphitia102 said:
Any brand that you recommend? Is there one fluid that's better than the other?

I use Amosil Severe Gear 75w-90 in mine. Its only about 10 or 11 bucks a qt but since I'm a preffered customer, I can now get it for around 7 or 8 bucks a qt.

Note: Redline, Amsoil, and Royal Purple are all in pretty close competition with one another so its up to you. I'de stick to a well known synthetic brand though like the ones I just mentioned.

Seraphitia102 · Jun 17, 2005

Dark Knight GT said:
I use Amosil Severe Gear 75w-90 in mine. Its only about 10 or 11 bucks a qt but since I'm a preffered customer, I can now get it for around 7 or 8 bucks a qt.

Note: Redline, Amsoil, and Royal Purple are all in pretty close competition with one another so its up to you. I'de stick to a well known synthetic brand though like the ones I just mentioned.

My manual says to use 75W140 so is using 75W90 okay?

Dark Knight GT · Jun 17, 2005

Seraphitia102 said:
My manual says to use 75W140 so is using 75W90 okay?

Yeah, it will be fine. Its what I use in my setup. If you wish to use 75w-140, then that will be fine as well. If your car is still under warranty, then I'de use 75w-140, as that is what the manual calls for. Either way, you're not going to hurt anything.

75W140 gear fluid, change?

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