r-mm

I think I bought the wrong stuff... the naming of the Redline 75W90 always throws me and I bought the 75w90 WITH friction modifiers.
I believe this is not ok for the manual transaxle and 75w90NS is needed. In fact I have an old bottle of 75w90NS and it mentions Porsche Transaxles on it.

The open front differential I am not sure about. I believe I'm reading that either product will work and if anything the 75W90 is in fact the suggested product.

Says Redline:

75W90 GL-5 Gear Oil

• Most popular Red Line gear oil, with thousands of applications for passenger cars, light trucks and racing vehicles
• Contains additional friction modifiers for suitability with clutch-type limited slip differentials - for most LSDs, no additional friction modifiers are required
• This product is not designed for use in most manual transmissions or transaxles in passenger vehicles, since the extreme slipperiness may cause synchronizer mesh issues that lead to shifting problems

RyanH.

NS is the "correct" version. I'm not sure what happens with the other version but NS is the one everyone seems to use:

https://rennlist.com/forums/964-foru...-is-magic.html

r-mm

I am quite sure 75w90ns is correct for the transaxle.

Can anyone help me confirm 75w90 is correct for the front differential ? Redline certainly makes it sound that way.

LM964

SWEPCO 201 Multi-purpose Gear Oil 80W90 is all I have been using for any of the C4's I've had - front and rear diff/gears. Certainly not the cheapest but up there in terms of quality.

However, I would have thought in your case 75W isn't going to be a showstopper by any means - arguably maybe even the better viscosity for the front diff. Someone else with more gear oil experience may chip in.

r-mm

The factory lit is quite clear on viscosicty - 75W90 (unless this has been superceded by some tech bulletin I'm unaware of).

Its the NS vs non NS redline formulation I was asking on.

-nick

The "NS" does not have LSD friction modifiers, which is what you want for a C4. I've switched back and forth between Redline, Swepco, and Delvac. Swepco is nice in the summer but gets pretty thick in the winter.

edit- Redline MT-90 is a GL-4, good for the 944/Audi transmissions, not for 915/G50.

964Lovac

Good reminder that I need to put my 964 up on jack stands and get this and some other maintenance items done.
Nick, would you suggest Mobil 1 Delvac instead of Redline? I see a lot of other 964 owners using that, is that good for C4?

Goughary

I have delvac in my C4. I absolutely hate it in the cold. Not bad in the summer.

Over all i preferred the redline.

r-mm

My C4 came to me with no real maintenance history so I don't know what's in the transaxle or how long its been there, but I will say in current New England temps it balks into second for the first few shifts of the day. When my 75W90NS arrives I'll let you all know if its an improvement.

964Lovac

Thank you, I will be purchasing this then!

It does tend to get "cold" in Atlanta, but nothing like the northeast. I too don't know what is in the transmission in terms of oil so figure it is time to replace!

-nick

To summarize: the factory manual calls for a 75w90 GL-5 oil with no LSD additives.

Outside of that:
- GL-5 has been Porsche's recommendation for all 911s since 1976. Before that the recommendation was MIL-L-2105, which is essentially the same as GL-5. The Audi transaxle cars (944 series) called for GL-4.
- G50/G64 synchro and gear issues seem pretty rare and people have been using all kinds of different oils in them. So it seems that you can't really choose a wrong oil as long as it's a GL-5 and 75w90 weight.
- According to Matt @ Guard Transmissions, the additive is only necessary if you're hearing lsd chatter and are annoyed by it and don't care that the additives makes the lsd less effective. Porsche has never called for non-slip oils or additives in any 915 or G50/G64 variant. Some oil do create more lsd chatter than others, ie, they're less slippery.

In my experience, the synthetics shift better when the transmission is cold, <30F, and don't feel so thick and gummy at very low temperatures. After the trans warms a little, they all seem pretty similar. I'm using Mobil Delvac 1 75w90 at the moment and have no complaints. It's supposed to be one of the less slippery oils, so spins the synchros up a little quicker. Honestly doesn't feel much different than any of the other oils that I've used.

JasonAndreas

Porsche pulled the GL-5 recommendation for the 964 and 993 G50/G64 (but not the pre-964 G50) back in 1998.

-nick

Ooh, is there a TSB on that Jason? I don't think I've noticed it. I've got a pre-3.2 workshop manual that lists GL-5 for '76- and the MIL oil for prior years of the 915. There's a TSB in '88 warning not to use GL-6 or GL-7 for fear of synchro failure. That's all I've seen for recommendations.

Laker

I still use the Delvac, because it's much easier to find... i just take the first few shifts easy in the cold, during the first 2 minutes.

JasonAndreas

I have and would have linked to it in my message. But I am not able to access the upload/download part of the Rennlist Member page sites where I keep everything posted for here. I'll attach it to this message. There was an additional dealer note from PCNA circa 1998 but I can't find it locally at the moment.