928 at last

Hi All,
Couple of things I need a hand with, first, is there a sustantial difference between 80/90 and straight 90 weight gear lube? and, what's the preferred option for my unit? (Toronto Porsche dealer said definetly don't use synthetic). Second, I'm finding Castrol Syntec 20/50 motor remarkably difficult to find around here (I'm just north of Toronto about 20 miles), any thoughts on where I can find some? (Same problem with Castrol GTLMA DOT 4 Brake fluid).
Thanks!

Ed Scherer

FWIW, I've been running Castrol Syntec 75W-90 (full synthetic) in my '90 S4 gear box for a few years. I'd be curious why this dealer/shop said not to use synthetic.

928 at last

Ed,
Dealer said it would screw up the shifting and PSD operation. I know you can run that grade in a manual shift though?????
Thx.
Rob.

Tails

I been running synthetic in my transaxle for nearly 2 years with no problems with PSD operation. I cannot understand how a synthetic oil would effect the action of the 100% disk clutch lock mechanism of the PSD.

I also cannot understand how it would affect the operation of the gear changes in a auto box as they are oil independent of one another, that is ATF in auto box and Gear Oil in transaxle.

Tails 1990 928S4 Auto

928 at last

Hi Ed,
What you're saying make sense to me. And although I've worked on some fairly "exotic" cars, I'm new to the 928 Porsche. You would think that there wouldn't be much difference between the systems (auto/manual) as they relate to the differential, and that the two systems are separate. I'd be perfectly happy running a full synthetic in the car. (And prefer to). If you've had no complaints, I'm thinking of returning the lube I bought, and going back to my first choice. Any comment on the weight ?
Thanks,
Rob.

atb

Rob,

You'll be fine with the synthetics.

I've run 75w/90 (with a mix of NS) Redline for about 7 years without any issue.
I've run Redline and Mobile 1 ATF for the same without any issues.

To be honest, I've never heard of anything but improved performance with them from 928 owners that have used them.

Ed Scherer

To be fair, it's Tails that offered the (correct) observation that the diff and tranny are separate on the AT.

I've been happy with the synthetic, and I don't think I've heard of anyone who wasn't.

I'm pretty sure I researched this when I changed my diff fluid a few years ago and determined that 75W-90 was the right thing.

You might want to just search this forum ("AT", "75W-90", "diff", whatever) and read up on it.

Also, FWIW, I'm running Mobil 1 synthetic ATF for the AT fluid. And it's been just fine. I think a lot of others have reported similar results.

Ed Scherer

Also: for all other fluids, just search this forum. Most of these topics have been beaten to death on here (motor oil choice is a "religious war" type thing, in particular).

If the service history on your PSD is unknown, you should probably consider replacing the brake fluid on it, too. There are good writeups on that procedure, if you haven't already found it. Hmmm... PSD service is actually something I need to do on my car ASAP! Long overdue. )

928 at last

OK., Thanks for the responses!!!!
I'm going with the 80/90 full synthetic.
It's a summer car only anyway.
As far as the PSD, I know I've gat a problem there, and I've got hoist time booked for this Sunday and the Tech printout ready to go. My only remaining problem is finding the Castrol GTLMA DOT 4 that the forums are recommending.......
Soon,
Rob.

928 at last

OK, found the GTLMA after two calls to Castrol in US and Canada. It was actually available within a 2 mile radius of my house!!!?????. Looks like we're good to go.
Thanks again to all who responded.

jon928se

check that your 80W90 Synthetic is also GL5. There's a lot of 75W90 or 80W90 around that is GL4 and GL4 ain't good enough for the 928 Transaxle.

928 at last

Jon,
It's apparently GL-5 +
Thx!
Rob.