Amsoil 75W90 Synthetic in the differential
12-22-2011, 12:48 PM
#21
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I had this same question which one to use 75w90n or 110 and i ended up going with amsoil also buut with the 110 because i drive my car hard on the mountains and also live in GA and have 4.77 gears, so i had a lot of noise with 75w90 and after hot days and hard driving it became a bit notchy, but with the 75w110 its butter smooth and the whinne got cut down by at least half, im planing on changing mine every 15k just bc of the way i drive the car hope this helps you out bro
12-22-2011, 06:21 PM
#23
It seems that very few people have substantive evidence on when the fluid starts to go bad and needs replacing. Because of this, most folks err on the side of caution like you do. I do find it a bit excessive to change it "far below" 15k miles when Amsoil (who has a vested interest in protecting their customers) recommends a 50k mile change interval under "severe duty" conditions.
I only drive this car about 7-8k miles per year. So long as the change interval is legit, I could go 5 years without changing the fluid and still be ok.
I only drive this car about 7-8k miles per year. So long as the change interval is legit, I could go 5 years without changing the fluid and still be ok.
12-24-2011, 02:00 AM
#24
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Next time do this test right before you change diff oil:
Find an empty parking lot big enough to drive around in circles - not at full steering lock but (let's say) twice the diameter.
First drive in a straight line & steady speed and "enter" the circle.
Pick a speed so you can easily "manage" the circle.
Let the car settle - for about 1/4 circle - and then slightly accelerate.
Don't make the rear step out - drifting is fun but not what you want here.
Feel what the car does.
Does the front want to go to the outside of the corner?
Like FWD understeer?
This will NOT be because of the front loosing grip but most likely because the inner rear wheel wants to go as fast as the outer.
Slowly reducing throttle will make the front turn-in again - also like a FWD.
Now change your diff oil and do this "test" again.
Did the behavior change?
If so, ask yourself: Am I using the right diff oil?
The OEM spec is single grade SAE 90 GL-5/6.
Not a multi grade.
The closest to OEM spec is LE-1605.
Second is any 75W-110 (most likely Amsoil)
A bit thicker is any 75W-140.
Diff oil should not only protect your final drive but also make the Torsen LSD work the way it has to.
I've asked Torsen about the influence of oil on the behavior (mainly the torque bias) and the replyed it would have an influence but it would not be noticable to the common driver.
Put a Torsen in a car in which one can feel 5 psi of tyre pressure difference and ask yourself: would I be able to feel a torque bias difference because of diff oil?
My answer: YES!
When both wheels have equal grip but they DO have different revs - like in the "test" - the Torsen relys on oil to allow those differences in revs.
An oil that can not handle the extreme pressures (anymore - or not at all) will make the Torsen lock-up more than it should and the inner wheel at lower revs will want to rev as much as the outer wheel, resulting in unstable behavior.
I've noticed this a couple of times in my car, with an OEM diff and my 4.57's.
A diff oil change - and nothing but - made the car handle like I was used to.
Find an empty parking lot big enough to drive around in circles - not at full steering lock but (let's say) twice the diameter.
First drive in a straight line & steady speed and "enter" the circle.
Pick a speed so you can easily "manage" the circle.
Let the car settle - for about 1/4 circle - and then slightly accelerate.
Don't make the rear step out - drifting is fun but not what you want here.
Feel what the car does.
Does the front want to go to the outside of the corner?
Like FWD understeer?
This will NOT be because of the front loosing grip but most likely because the inner rear wheel wants to go as fast as the outer.
Slowly reducing throttle will make the front turn-in again - also like a FWD.
Now change your diff oil and do this "test" again.
Did the behavior change?
If so, ask yourself: Am I using the right diff oil?
The OEM spec is single grade SAE 90 GL-5/6.
Not a multi grade.
The closest to OEM spec is LE-1605.
Second is any 75W-110 (most likely Amsoil)
A bit thicker is any 75W-140.
Diff oil should not only protect your final drive but also make the Torsen LSD work the way it has to.
I've asked Torsen about the influence of oil on the behavior (mainly the torque bias) and the replyed it would have an influence but it would not be noticable to the common driver.
Put a Torsen in a car in which one can feel 5 psi of tyre pressure difference and ask yourself: would I be able to feel a torque bias difference because of diff oil?
My answer: YES!
When both wheels have equal grip but they DO have different revs - like in the "test" - the Torsen relys on oil to allow those differences in revs.
An oil that can not handle the extreme pressures (anymore - or not at all) will make the Torsen lock-up more than it should and the inner wheel at lower revs will want to rev as much as the outer wheel, resulting in unstable behavior.
I've noticed this a couple of times in my car, with an OEM diff and my 4.57's.
A diff oil change - and nothing but - made the car handle like I was used to.
12-24-2011, 04:36 AM
#25
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Next time do this test right before you change diff oil:
Find an empty parking lot big enough to drive around in circles - not at full steering lock but (let's say) twice the diameter.
First drive in a straight line & steady speed and "enter" the circle.
Pick a speed so you can easily "manage" the circle.
Let the car settle - for about 1/4 circle - and then slightly accelerate.
Don't make the rear step out - drifting is fun but not what you want here.
Feel what the car does.
Does the front want to go to the outside of the corner?
Like FWD understeer?
This will NOT be because of the front loosing grip but most likely because the inner rear wheel wants to go as fast as the outer.
Slowly reducing throttle will make the front turn-in again - also like a FWD.
Now change your diff oil and do this "test" again.
Did the behavior change?
If so, ask yourself: Am I using the right diff oil?
Find an empty parking lot big enough to drive around in circles - not at full steering lock but (let's say) twice the diameter.
First drive in a straight line & steady speed and "enter" the circle.
Pick a speed so you can easily "manage" the circle.
Let the car settle - for about 1/4 circle - and then slightly accelerate.
Don't make the rear step out - drifting is fun but not what you want here.
Feel what the car does.
Does the front want to go to the outside of the corner?
Like FWD understeer?
This will NOT be because of the front loosing grip but most likely because the inner rear wheel wants to go as fast as the outer.
Slowly reducing throttle will make the front turn-in again - also like a FWD.
Now change your diff oil and do this "test" again.
Did the behavior change?
If so, ask yourself: Am I using the right diff oil?
Good post Spitfire
12-24-2011, 07:01 AM
#26
I've always believed that diff fluid viscosity affects lock-up action of our diffs on this vehicle. I've noticed it many times while using 75w90, 75w110, and 75w140. SpitfireS and I seem to be the only ones who have noticed this.
12-25-2011, 10:24 AM
#30
Moderator
I exclusively use mobil1 75w90 in all my customers S2k and mine. Drains super clean after 30k miles. I have plenty of drifting clutch dumps and track time on my Dif. Highly recommend it.