JWise

In the last 2 weeks I've had the opportunity to run 3 different fluids in my PSD. As far as shift quality, I had the following observations, nothing too surprising:

• Swepco Blue - this had been in the car since right after I purchased it four years and 10K miles ago. Shifted smoothly, even when cold.
• Redline 75W-90 NS - Sean and I put this in a couple weekends ago. Seemed "notchy" compared to the Swepco, even when warm.
• Mobil 1 LS 75W-90 - Just swapped out today due to displeasure with the Redline notchiness, along with Greg Brown's strong recommendation for the Mobil 1 LS. Seemed smoother than the Redline, though maybe not quite as smooth as the Swepco.

Now this is where it gets weird - I swear I could tell a noticeable difference in handling between these three fluids in the diff. Both the Swepco and the Mobil 1 seemed to result in less responsive handling - with Redline in the diff the car seemed to be much more eager to rotate, both at initial turn-in and especially through the turn. Am I imagining things, or has anyone else noticed a handling difference between these popular brands of differential fluids?

SeanR

I looked at your SMS today Jarrod, and had to read it thrice to see if I was actually understanding what you were talking about. I'm understanding it better now and have about the same response. "I dunno"

Next, let's try that Walmart Gear oil and see how it works

JWise

Yes, in case my above description is unclear, let me clarify - with Redline, the car seemed to push less, the rear was more lively and willing to swing around, "yaw" if you will. Not to the point of breaking loose oversteer, just eager to help rotate the car through the turn.

Oh, and I'll pass on the Wally World gear oil.

AO

I don't think you are imagining things. The RedLine NS is supposed to help limited slip cars. When you're in a turn (only when that little green light comes on) the limited slip kicks in and provides limited slip to the inside (or is it outside?) wheel. The gear oil you have in there can have an impact on the "bite". But like you experienced, it can also affect the feel of shifting.

personally, I've been running RedLine NS for some time now and am generally pleased with it. It is quite notchy when cold, but smooths out after it warms up. I've not tried anythign else, but will probably go with Mobil one next time just because it's much easier to source than the RL.

IcemanG17

I also think its possible....it is continuously variable system in terms of lock up and LSD's do change the way cars handle....it appears that open diff cars handle the "best" & the tighter the LSD's lock up is the more if will effect how much the car pushes (seems backward but true) when off throttle....

I do think low lockup LSD's don't make much of a change...at least I haven't noticed it in the Estate yet..since its stock lockup....

I also don't know how fast the PSD reacts when off throttle....I would imagine its pretty quick..it could take the idle switch from the TPS to know....

JWise

All I know is I liked how the handling felt with the Redline. With the Swepco, and to a lesser degree the Mobil 1, it felt heavier and more ponderous - like it didn't really want to change directions. With the Redline NS, the eagerness to change direction was there, making it feel lighter and livelier, like it was really dialed in.

GregBBRD

I'd wait a few hundred miles...and a lot of turning to judge the change. It will take at least that long for the old fluid to get out of the PSD discs and get the new fluid in...

RKD in OKC

Could it be you jacked up the rear to change the juice and it hasn't settled back down yet?

AO

Actually it uses accelerometers in the brain box under the driver's seat.

Tom in Austin

Uses both an accelerometer (under the driver's seat) and the ABS wheel speed sensors. Was just reading up on this the other day (WSM Vol 3) and the test procedure for the accelerometer signal requires lifting the driver's side 15 deg, so it sounds as if the primary input is/are the wheel speed values.

JWise

Good point. Hadn't thought about that, but have experienced it before on a Ford LSD that was chattering - it took a brief period after adding friction modifier before the chatter went away. I have less than 15 miles on the Mobil 1 so I'll give it some time.

Yes, I jacked up the rear but it settles quickly - unlike the front. Visually, the ride height appears proper.

fastduck

JWise,

The NS formulation of gear oils isn't intended for most limited slip differentials until you add bits and bits of their friction modifier to yield the right amount of slipage. I'm not surprised that it felt different than the other fluids. I talked to them bunches over what to use for my viscous-coupling diff. on my twin turbo, and in that case, lacking clutches, NS was finally determined. The "plain" 75-90W is what they told me to use for the 928 (LSD differential only, Auto trans.)

"If" I understand it, the NS without modifier would lock-up the clutches more than the other lubes.

I could be full of it,
John