RyanPerrella

Rob,

I have only 1500 miles on my car, i havent bothered to check my trans oil honestly, never really thought i should. But I will tell you the car shifts great, a little notchy when cold, but other then that its smooth. I wouldnt mention the smoothness if i didnt think it. Before buying the 928 i had figured the shifts would be sloppy and clunky but not so. I think its a great trans. Also though i never drove the car before i changed the fluid so i cant give you an honest before and after redline story.

From what i have read i would in the future get 3L of the 75/90 and 1L of the 75/90NS, I have all NS based on my last post on this thread, i had forgotten! But as for long term i cant tell you if its any good but so far i like it yeah, as much as i can like a gearbox oil. But if i were doing it again today i would go with the 75%-25% mix like mark noted.

And remember there are 2 drain plugs one in the front and one in the rear of the case.

Good luck

Doug Hillary

Hi,
some gear oils do perform better (or worse) than others!

There are seven (7) gear lubricant specifications (API - USA)
GL1 - Straight mineral oil
GL2 - Usually of a fatty composition (for worm drives)
GL3 - Contains mild Extreme Pressure (EP) additives
GL4# - Contains a prescribed level of EP additives (MIL-L-2105 A spec) - for spiral bevel and hypoid gear but without shock loading!
GL5# - Contains higher level of EP additives (MIL-L-2105 B/C/D and etc) - For hypoid and other gears in the severest of loading. And for use in some manual gearboxes as in the 928
GL6 - Obsolete (was for high pinion offset hypoid gears)
MT1# - for manual transmissions in trucks (usually as an SAE50 oil)

# These lubricants can be of mineral or synthetic form

Some synthetic gear oils now meet both GL5 and GL4 operating specifications - these are actually GL5 oils with a less aggressive additive package performance
Many gear oils are especially made for the gear set of a specific manufacturer (Mitsubishi, VW, BMW etc) and for very very sound reasons.

Synthetic GL5 gear oils offer extended benefits over their mineral cousins. It is always best to change from a mineral to a synthetic GL5 gear oil within the first 5k from new or rebuild. Changing later CAN lead to the sulfur/phosphorous EP additive build up on formed on seals etc being removed and seepage occuring. I have done this many times over the years and NO seepage has ever occured

Synthetic GL5 gear oils run cooler (up to 20C), last longer and are more thermally stable at wide extremes of temperature than mineral GL5 lubricants. They also greatly extend seal and gear life due to their inherent characteristics

The constant Porsche recommendation has been to use a 75w-90 (SAE90 is superceded) GL5 oil and that specification is for all 928s. And it is specified for the Auto's differential too of course

As mentioned above all GL5 gear oils contain a shock proof ingredient. Great care should be taken if adding "extra" additives. These can destroy synchroniser performance (some may slightly "improve" it too)

I have used both Castrol (Syntrax) and Mobil 1 GL5 75w-90 (and MT1) gear oils for the last 10 years in a variety of applications with excellent results. Both have performed at the same level and out to 1m kms (620k miles) oil change intervals monitored by used oil analysis. In some applications the temepratures were constant at 100C+
Synthetic GL5 gear oils can be used up to 120C (250F) constantly and to 150C (300F) intermittently. Mineral gear oils deteriorate rapidly after about 110C.

Most manual 928s in normal use would see operating temperatures around >100C and Auto diffs >90C with synthetic GL5 lubricants

Very expensive gear oils may NOT produce any significant benefit - a synthetic GL5 lubricant from the major Oil Companies (Shell, Castrol-BP, Mobil, Motul, Chevron etc) will all do the job well in a 928

I hope this is of some assistance

Regards

worf928

My understanding is that the PSD is NOT a 'normal' limited slip differential. Without the PSD pump, clutch pack, etc. it is an open diff. I seem to recall having my '89 and '91 on stands at the same time and confirming that the '89's rear wheels roll in the same direction when turned by hand and that the '91's wheels turn in opposite directions.

Can anyone confirm or refute?

Rob Edwards

Guys- thanks for the responses! My father ( a lifelong gearhead who was never in a position to buy a dreamcar, he was too busy paying my tuition) is coming this weekend, so I'm hoofing it to make the car as nice as possible for his visit. He still can't believe that I bought one....

BarryW

I ran Amsoil 75w-90 for about 15k miles. It worked very well, smooth shifts. I added no friction modifier and on a very few occasions the (LS) differential seemed to lock up making the steering very heavy for a bit at slow speed, i.e. in a parking lot when hot. I recently changed it. It looked fine. Pretty much like it looked when I put it in except darker. I put Redline 75w-90 with a half dose of friction modifier in it and it still shifts smoothly and no sign of dragging. Not very scientific but....hey.

86.5 5spd w/LSD

atb

I just had an 928 Owner ask me about this. He states that he filled his '82 with Redline MT-90, only to find out that the label on the bottle gave the following warning:

"This product is not designed for use in a rear-wheel
drive differential which requires a GL-5 lubricant. The
extreme stability of this product allows use for extended
periods."

Why would it not be suitable for the differential if it works for the tranny?

Any ideas? Anyone running Redline MT-90 have any issues?

Doug Hillary

Hi Adam,
MT90 is a "gearbox" only GL4 lubricant that can be used in GL1 and GL3 applications too. Some gearboxes specify engine oil (GL1 & GL3) and MT90 "fits the bill" if a SAE40 or 15w-40 engine oil is called for as well. It will have little if any "shock loading" abilities

It should NOT be used in any transaxle unless a GL4 lubricant is called for!

Why is it unsuitable? - It is "all" to do with the additive package (and sometimes the base oil too) and finally the EP/load carrying/shock loading characteristics of the total package

If a GL4 lubricant is used in a GL5 application it is possible to have excessive and rapid hypoid gear tooth wear and/or corrosion of certain types of metallurgy used in the component concerned

Adam, as we have seen here over the years some people "get carried away" by the advertising hype of certain products and Brand names. It is more important to use a lubricant of the correct specification and viscosity - the Brand is really almost insignificant!

Sadly some Techs at the Oil Companies often lead to more confusion due to their lack of training and/or Technical and application knowledge. It is certainly this way in Australia!

Regards

heinrich

AMEN Brother! Amen ....

H2

Yea...I once bought 8 qts of MT-90 Redline before I realized I needed the NS GL-5 stuff. What shall I do with the MT-90? Run it in my Chebby Pickup?

H2

GlenL

Hint: 944s take GL-4

Garth S

A similar debate on which GL-5 is best suited to Porsche transaxles was held on the 993 board: In the Mobile family, it was recommended by some that 'normal' syn Mobil 1 75W-90 is well suited for differentials; however, if syncros and gears are involved, Mobilube 75W-90 SHC is a better choice ..... this product is apparently superceeded by Mobil DELVAC 75W-90.

There is some info here ....

davek9

I am reading the label on the 1 qt bottle of Mobil 1 75w-90 Synthetic Gear Lubricant and it states that it is for use in both Conventional and Limited Slip applications, it also states that it exceeds the performance requirements of API Service GL-5.
I too have heard conflicting views over the past years on what should be used.
Guess I picked the right one, after reading this its finally going in!
It is also good to know others have used it too w/no issues.

This is a GREAT post, thanks Doug H. for the detailed info!

atb

Here, here!

Doug Hillary

Hi,
davek9 - Adam, my pleasure

GlenL - whilst the 944s require a GL4 gearbox lubricant, the 944S2 and 968 have a GL5 requirement as I understand it

Garth S - You are correct, Mobil 1 SHC 75w-90 (which I have used extensively) has been superceded by Mobil DELVAC Synthetic Gear Oil 75w-90 - at least in NA. It appears to retain the same characteristics, it is a GL5 lubricant, and the change of name is probably just to expose the Commercial DELVAC name more

The DELVAC version will have the "higher" specification (than the Mobil 1 LS version - see below) to cover elevated EP and greater shock loading requirements amongst others - it is a very well specified lubricant for Commercial use. It will work well in most GL5 applications

Mobil 1 Synthetic Gear Lubricant LS 75w-90 (which is suitable for both limited slip and normal applications) has a slightly different formulation than the DELVAC lubricant
In particular it is significantly thinner when cold and slightly so at 100C.
It's higher Viscosity Index indicates a lessor overall change between cold and hot

Be careful if selecting a Commercial lubricant of any Brand as some may not be suitable for your intended application. As an example DELVAC make engine lubricants unsuitable for petrol engines and many of the DELVAC products are manufactured from mineral base stocks. This applies to most major Oil Companies products

It is always wise to follow the component Manufacture's lubricant recommendations today - especially in regards to viscosities - as there are many variations of what were once very simple recommendations. And the supersession of commonly known products is rife

Regards

Manfred

Thread revival.

So is that 75%NS and 25% standard or the other way around? I'm changing out my 5spd PSD ('91GT).