kevin2012

As for 91 S4 with PSD, do I need 75w90 LSD fluid or non LSD fluid for the diff? The owner's manual didn't mention anything about using LSD fluid. Some say LSD but some say it does not need it. I am very confused.

Thanks for the advice.

dr bob

The PSD does not need or want LSD fluid. The "friction modifiers" in LSD fluid are there to manage LSD differential clutch chattering in slow corners. Meanwhile, PSD engages only when slip is detected, so slow-corner differential clutch chattering doesn't happen. The PSD actally wants more clutch friction when it engages.

kevin2012

Can it cause any damage to the diff in long run if LSD fluid is used? I have been using LSD fluid for few years.

Cheers.

worf928

More important than what oil you put in the gear box or how often you change the oil in the gear box is:

You have to flush the PSD hydraulics - this is not the gear box - every two or so years. It's brake fluid and an open-to-the-atmosphere system.

dr bob

Shouldn't really hurt anything, since the PSD just keeps adding hydraulic pressure to the PSD slave as it tries to solve the detected slip.

But as Dave reminds us, don't forget the fluid service on that system. Considering where it lives, I'd be looking for a GM-style reservoir boot to put under the filler cap. Seems like a lot of road crud and moisture would find its way in with a vented cap on there.

GregBBRD

Three significantly different kinds of friction discs in the PSD, over the short period of time they were used.

I'd run 75/90 Mobil One LS in your '91....the discs are pretty severe and the friction modifier will work very well to make them engage and release smoother.

It's taken Porsche years to figure out that the smoother a limited slip engages and releases, the faster the vehicle will be through a corner....something successful racers have known for years.

GregBBRD

The PSD units have a hose attached to the vented cap, which runs downhill...you pretty much have to submerge the car, sitting on one of the sides, to get crap into the reservoir.

worf928

Yup. But, over the years the brake fluid will absorb moisture through the vent line and turn to lemon (or spearmint) Jello. I'm sure you've see that on too many occasions.