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3 different 75w/90 gear oils, are they all similar enough?

3 different 75w/90 gear oils, are they all similar enough?

3 different 75w/90 gear oils, are they all similar enough?

Hey all,

Having an issue trying to determine if gear oil C is similar enough to be a replacement for A and B. Oils A & B are both Porsche Approved for use in their trans axles and differentials but they are generally difficult/expensive to purchase. All 3 of them are GL5 rated but Porsche does not specifically ask for GL5 they only want you to use oil A & B.

Oil C is recommended by it's own manufacturer for use in Porsche transaxles & differentials. Below I am listing the data/specs that I could find for all 3 and I have looked at many other GL5 oils as well. It seems that the bigger difference with these three oils from other GL5 75w/90 gear oils is these 3 seem to have lower viscosity at lower temps while still having higher flash points than others I have seen. Also the two approved oils A and B seem to have a lower Brookfield -40 C Viscosity than oil C as well as other oils I have looked at.

How big of a difference would this Brookfield/dynamic viscosity make in actual use. Is oil C close enough that it could be used in place of A & B?

I appreciate any and all feed back.

Thank you in advance!

SAE Grade 75W-90
Viscosity, cSt at 40 ºC 77.2
Viscosity, cSt at 100 ºC 14.5
Viscosity, cSt at 0ºC (measured) 590
Viscosity Index 197
Brookfield Viscosity at -40 ºC, cP 20,000
Pour Point, ºC -48
Flash Point, ºC 212
Copper corrosion, 3 hr at 150ºC 1B

SAE J 306 75W-90 SAE Viscosity Grade
0 2 ISO 3104 81 Kinematic Viscosity @40 C mm /s
0 2 ISO 3104 14.9 Kinematic Viscosity @100 C mm /s
ISO 2909 194 Viscosity Index
ISO 9262 35000 Dynamic Viscosity mPa s
0 3 ISO 12185 879 Density @15 C kg/m
0 ISO 2592 205 Flash Point (COC) C
0 ISO 3016 -45 Pour Point C

Specifications and approvals:
API GL5 · MIL-L 2105 C · MIL-L 2105 D
Technical data
SAE class (gear oils) 75W-90
SAE J306
Density at 15 °C 0,855 g/cm³
DIN 51757
Viscosity at 40 °C 80,9 mm²/s
ASTM D 7042-04
Viscosity at 100 °C 14,3 mm²/s
ASTM D 7042-04
Viscosity at -40°C (Brookfield)
<= 150000 mPas
ASTM D 2983-09
Viscosity index 184
DIN ISO 2909
Pour point -60 °C
DIN ISO 3016
Flash point 190 °C
DIN ISO 2592
Color number (ASTM) L 1,0
DIN ISO 2049

RE: 3 different 75w/90 gear oils, are they all similar enough?

Are you going to operate the vehicle at low temperatures? Using oil C you would need to warm the transaxle slowly on cold mornings. Synchromesh operation will also be poorer until warmed up.

je suis charlie

RE: 3 different 75w/90 gear oils, are they all similar enough?

the difference between oil A and B on the one side and oil C on the other side may be more then just the difference in Brookfield viscosity at very low temperatures. The difference in viscosity may well come from the various base oils that are used to formulate the final product. oil A and B most likely will include a larger amount of PAO (poly alfa olefins) that both guarantee better fluidity at low temperatures and most likely a somewhat higher viscosity at actual operating temperatures which might translate into better protection against wear. they also may contain additives that not necessarily meet typical API GL5 requirements, but do fulfill the specific test requirements of the vehicle manufacturer, both in terms of wear protection and shifting speed - typically with API GL5 gear oils longer shifting times may be experienced together with higher wear in synchromesh mechanisms.

Porsche is known to be very clear in specifying which lubricants to use since they have experienced that oils that do meet requirements of main stream applications not always are up to scratch when used in their vehicles. therefore i would stick to the oils approved to minimize the risk of (very) costly repairs in future in this case.

the approved oils may be costly too, but you only need a few litres and they will not need to be changed that often.

RE: 3 different 75w/90 gear oils, are they all similar enough?

Based on my experience with Porsche products in the past, I would highly recommend that you follow their recommended fill. Yeah, it's expensive, but you're not often doing R&R on trans or diff oil.

If this car is PDK or DCT, then you have even less choice- those cars should see factory fill and factory fill only.

RE: 3 different 75w/90 gear oils, are they all similar enough?

Hey all,

Thanks VERY much for the replies.

Gruntguru, actually no the vehicle(s) won't see cold temps (I am in South Florida).

romke, this is what I was mainly concerned about; that the warm/running viscosity of all three are pretty close but the 2 approved oils have a very different cold viscosity my thought was that there is probably more to it and and that there is a reason(s) in this particular case that no other oils are approved.

jgKRI, car is a manual trans, I have no problem at all paying more for the correct approved lubricant and I have always lived by this rule especially in high end cars like this. The only reason I even bothered to look at other oils was because many very large Porsche specific supply houses and Porsche specific repair shops not only recommend but install in their customers' cars non-approved gear oils in these same applications that I am looking at. It was only after I looked more closely at the data sheets that I realized in colder tests only the approved oils seem to have this property which is why I came here looking for more opionions. shadeshappy

BTW, oil C was the only one that I found after really looking that actually comes close to A & B in the warm/operating temps. The other oils that I have seen recommended and/or used by Porsche specific shops are even higher viscosity across the board! No wonder some folks are complaining about notchy shifting,especially when cold ponder

Again, I want to thank you all for the replies. You have confirmed what I was thinking and made my decision very easy. I will only go with an approved oil for this application. Thanks!!!

RE: 3 different 75w/90 gear oils, are they all similar enough?

Since you live in a warm climate, the viscosity and VI of oil C will not produce any discernible performance effects. Even at say 0*C, the difference in oil temperature between oil C and oil B to produce the same viscosity, would be less than 5*C.

je suis charlie

RE: 3 different 75w/90 gear oils, are they all similar enough?

Thats what I was thinking originally but when I started to look at all the datasheets and compared the two approved oils with ALL the others (about 6 different recommended (by 3rd parties) oils) ..the two approved ones seem to be the only ones able to keep the viscosity this low at such low temps. It would seem there is something different about the approved ones so I'm not really willing to take the risk. If Porsche would have simply specified gl5 or some other rating and the data sheets would have been closer for all of them, maybe. But I think I will be much more comfortable with the approved oils in this case.

RE: 3 different 75w/90 gear oils, are they all similar enough?

WRT viscosity only - Porsche have clearly specified a very high VI - for protection and synchromesh performance at very low temperatures. Many makers specify ATF for the same reason. Of course ATF is too thin for this application.

High VI is not all beer and skittles. VI improvers generally compromise other attributes.

je suis charlie

RE: 3 different 75w/90 gear oils, are they all similar enough?

gruntguru, so if I understand you correctly you are saying that it takes more additives for the oil to remain less viscous at the lower temps? and that this higher percentage of additives will decrease the lurbrication/protection ability of the product? Obviously this would have benefits for cars operated in colder climates but perhaps would be less beneficial for cars that will not see lower temps very often if ever?

Am I understanding your point correctly?

BTW, I have re-read the requirements by Porsche and they specifically mention that Viscosity, cSt at 0ºC must be less than 600. Not sure if this makes any difference to your point.

Thanks again for all the feedback.

RE: 3 different 75w/90 gear oils, are they all similar enough?

Using the viscosity extrapolation calculator Link, the viscosities at 0*C for each oil are:
A. 535
B. 577
C. 618

Excerpt from Link
"Unfortunately, viscosity index improvers do have some drawbacks. The primary disadvantage is they are susceptible to mechanical shearing. When referring to the slinky analogy, it is easy to imagine a stretched-out slinky cut in half by mechanical processes to produce two shorter slinkys.

As the additive is repeatedly sheared, it loses its ability to act as a more viscous fluid at higher temperatures. Higher molecular weight polymers make better thickeners but tend to have less resistance to mechanical shear. Lower molecular weight polymers are more shear-resistant, but do not improve viscosity as effectively at higher temperatures and, therefore, must be used in larger quantities."

je suis charlie

RE: 3 different 75w/90 gear oils, are they all similar enough?

the viscosity characteristics of a oil are both the result of the baseoils used and the VI-improver. when you use synthetic (usually PAO with a few percent of esters) as the baseoil, you will need less VI improver to achieve the required fluidity characteristics. also the pourpoint of synthetic basefluids tend to be lower then the pourpoint of mineral baseoils. VI improvers used in gear oils may have a tendency to shear under high loads in gears and as a result will loose their effectiveness. therefore, you get better viscosity characteristics when using high quality baseoils that need less VI improver - the original characteristics last longer.

another consideration might be high temperature use - driving on highways with high speeds for a long time. under those circumstances high temporary shear may occur effectively lowering the viscosity temporarily to a value that no longer is able to protect the gear surface sufficiently against wear. temporary shear only occurs when using VI improvers, using high quality baseoils without VI improver can avoid the phenomenon. in the SAE engine oil viscosity specifications there is used a special test to assess whether the oil withstands high temperature/high shear conditions at 150 deg C, about the operating conditions that may be expected in the valve actuation mechanism (cams, followers).

there are thus several reasons to limit the amount of VI improver in a finished product and the only way to do that is to start out with better quality baseoils - that are unfortunately more expensive due to more processing steps that are involved..

RE: 3 different 75w/90 gear oils, are they all similar enough?

I suspect that A, B and C are all PAO based synthetics. If that is the case, the lower VI of oil C could be attributable to a lower dose of VI improvers or it might use an unconventional additive package which compromises VI in favour of oxidation stability or film strength. In either of these cases oil C could in fact be a better choice for a vehicle being operated in Florida. In the absence of better information of course the best option is the factory approved product.

je suis charlie

RE: 3 different 75w/90 gear oils, are they all similar enough?

Thank you again everyone for your help. I also think that oil C could be a good choice for warm climate areas. I think for now that I will stick to one of the approved oils BUT I will continue to do research.

If I was to send off a sample of 2 of the oils to a service like Blackstone for an analysis would this information help us to make a better decision or would an analysis like this still not reveal the actual make up of the products and what makes them different?


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