Giantviper

WHAT GEAR OIL DO YOU RECOMMEND WEIGHT AND BRAND
DO YOU RECOMMEND SYNTHETIC
HOW MUCH OIL DOES IT HOLD

CO951

Dan87951

I use Mobil One 75w90 LS. LS is for limited slip applications. I have been using this fluid for almost 15 years in my cars. No problems.

bonus12

Redline MT-90. Excellent stuff.

T&T Racing

Is the application for street or race? Swepco makes one for street, 201 and for race, 210. I use 210 in my 944 NA for racing..very happy with the product. A gallon is sufficient.

mytrplseven

+1 on Redline MT90 but not sure if there's a problem with it in LSD's. I'm an old school guy and as such I don't trust synthetic lubricants in any part of these old cars. On another note, if you still have the transaxle in the car, Harbor Freight has a cheap hand pump that fits the top of the oil bottles that makes refilling the tranny a snap.

porscharu

I put in amsoil when I resealed the trans. Granted I have only driven the car about thirty miles since the change but so far so good. Less squishy feel on gear changes and actually goes into reverse without having to going into first then reverse. I will have to get more time with it in the spring for m final verdict, but so far nothing bad to say.

Noahs944

I use LUCAS 75w90. LSD safe too.

NoRush993/951

I used Royal Purple 75w90 and I have LSD. Works great. No issues.

JET951

It should be a GL5 75w-90, as most ( not all ) GL4's are not recommended for hypoid diffs & all road going Porsche's have hypoid diffs & being a transaxle the gearbox & diff oil are not separated & we have seen quite a few 951 diffs & 915( older 911 ) diffs destroy their crown wheel & pinions because the " oil film strength " of the GL4 that was in the trans was not up to it , sometimes a small part of the pinion tooth will give way & get caught in the gear set ( gearbox )& will instantly smash a gear tooth off

If you look in the owners manual ( that came with the car ) Porsche states = GL5

There is a reason for that

944crazy

I just had a look in the owners manual for my 951 and it states for a transmission with a differential (ie the manual box) that GL4 oil is to be used. It only recommends GL5 for the differential of an auto box.

That said, I put Redline MT-90 in my box, and now it is broken...might just be a coincidence though, as my ring and pinion are in good nick. Did Porsche change their recommendation to GL5 for the manual box as well at some point?

JET951

I have not got the spec books with me at home , but from memory the 924 auto ( seperate diff had SAE 90 which is a GL5 99% of the time anyway ( high viscosity & very high oil film strength ), the 924 & 944 auto trans diff needed it because it was tiny ( every bit helps )

And from very distant memory the only Porsche owners manuals that didn't have the 75W-90 GL5 recommendation was the early 944 series , but they mentioned SAE 80 GL4 , this has a much higher oil film strength as compared to most 75W-90 GL4 oils & was designed for hypoid use ( this was a Long Time ago ) I can barely remember it

The 75w-90 Trans oil ( viscosity) only became available around the mid 1980's & this is where confusion can & does set in

As you have already have noticed the Red Line( MT - 90 75W - 90) is NOT recommended by Red Line for most REAR wheel drive cars ( if not all ) and they mention not to be used with a Hypoid Diff , all road going Porsche's have hypoid diff's
Redline recommends their 75W 90 Gear Oil ( thats all they state )

Porsche recommended 75W -90 GL5 as soon as it became available in the 915 trans ( 911 ) & before that we had to put up with the thicker ( slower gear change 80-90 GL5) , same goes for the 928 5 speed ( 75w-90 GL5 from 1985 ) & to be retro used in the earlier ones as well

The early 924 trans & early 944 manual trans ( pre mid 1980's ) we used SAE 80 GL4 , that this had a higher oil film strength than a GL4 75w-90 because it was a mono grade oil ( this was all pre mid 1980's ) & from memory I think the early 944 owners manuals may have mentioned the SAE 80 GL4 ( mono grade ) , but by the time the 968 came out ( last of the 944's ) it( owners manual) just says to use 75W-90 GL5

At the Porsche dealership I was at in the mid 1980's we had already switched from the SAE 80GL4 to the 75W-90 GL5 because it gave better gear shifting behaviour as compared to the mono grade 80 grade GL4

The interesting thing is not all gear oils are the same ( like engine oils ) , there will be small differences , like two or three oil companies will market a ( say ) a 75W - 90 & all in this example will be GL4 , one company will concentrate on "easier gear shifting " , to do this the properties of the oil will be changed & or the oil viscosity base will be less viscous , so you end up with ( say ) three oil brands , all the same viscosity , but all with slightly different viscosities within the allowable range to call it a 75w-90 viscosity , so you end up with slightly different oils , but they are all are within the flexible guidelines that is a 75w-90

944crazy

Yeah my owners manual says SAE 80 API GL4 for the '86 Turbo.

So which brand of 75w90 GL5 do you use in your car, Bruce?

I'm asking because Redline says that their GL5 is not suitable for most manual transmissions as the extreme slipperiness may cause synchronizer mesh issues that lead to shifting problems. So I will need to change brands...

Swepco is a popular one that comes up in searches, but I can't find any here in Australia.

What about Mobil 1, or Royal Purple?

I know I'm opening up a can of worms here with brand preferences, but I just want to see what options are out there, and people's experiences with them.

Also, I've been reading that some types of GL5 may attack synchros. Can you shed some light here Bruce?

JET951

What a load of rubbish , GL5 attacking synhcros , thats all we have used from the MID 1990's , my own 968 alone ( 6 speed manual ) has had at least three owners , 270,000 Kms + ,( Millions & millions & millions of gear changes ) I have known the car since new ( hence why I purchased her ) and the 6 speed manual trans has NEVER been apart & has perfect gear shifting & no noises at all , she feels like a new 968 6 speed trans , so much for attacking sychro's , next thing I will hear it causes CLIMATE change

Sean in his 600HP 3.0L Twin cam 16V E85 951 5 speed 951 trans has been on GL5 since it was new ( 30 Years ) and apart from general maintenance we have no issues putting 600 Hp through her , its the GL5 keeping it together ( particularly the hypoid diff )

Think about it , we have NO choice , these transmissions need GL5 , ( High Oil Film Strength ), the old 1970's SAE 80 was OK in its day , but the 75w-90 GL5 ****s all over it

944crazy

Well, I'm just trying to be safe...just in case there are specific brands of GL5 that are not safe to use in our boxes. I'd rather pass on an 'internet myth' and have it be disproven by a professional like you, than just ignore it and hope for the best.

When I first did my gearbox oil change, I chose MT-90 because heaps of people recommended it across multiple threads on Rennlist and elsewhere, and because my own service manual called for a GL4 fluid. Even in this thread, MT90 was recommended multiple times, and you are the first person I've come across that has said it's no good.

If you are confident that GL5 is he only way to go, then I am inclined to believe you and go with that in my own box. You look like you have had great success with it. I'm not questioning your experience, I'm really just trying to avoid having any problems again.