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Old 04-30-2017, 03:19 AM   #1
grey wolf
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Default Motul competition 75w140 oil for rear diff (LSD)?

Ok, let me apologize in advance if somewhere in this forum, or on the internet altogether, there exists the information that I am looking for. I'm not a big internet user, so please don't crucify me for "not searching first" or whatever. I searched already, many different search terms, and haven't found the info I am looking for. So, sorry!

Here is my question. Has anyone used the Motul Gear Competition 75W140 in their rear diff with LSD? To clarify, this is the blue coloured fluid that is very expensive.

I have a 2001 JDM STI, that's the 6MT with an LSD rear diff. I'm going to change all the fluids, and I want to use all Motul lubricants, so it's basically between this stuff and the PA90 LSD oil I think. Unless someone can recommend anything better. I'm not looking for the best cheapest oil here, I am looking for THE BEST oil for my rear diff application. So, what do you all think?
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Old 05-02-2017, 01:56 PM   #2
Samurai Jack
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Just use the PA90. I don't know why you would want to use the 75W-140. That's pretty heavy stuff.

What are you going to use the car for?
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Old 05-02-2017, 02:11 PM   #3
The McYolo
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PA90 is what I used in mine and it's smooth as butter. 140 is pretty heavy to be honest. I wouldn't use it personally.
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Old 05-02-2017, 02:22 PM   #4
funk32
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only reason to use the 75w-140 would be if you are doing heavy competition. if you are just DD's and seeing a few track days I wouldn't use it.
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Old 05-02-2017, 09:43 PM   #5
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75w-90 for the transmission and 75w-90 is what is recommended for the rear differential, but if the vehicle is operated where ambient temperatures are normally at or above 32-F (0-C) and/or under more severe conditions, then 90 weight is to be used.

Use Motul Gear300 for the transmission and Motul PA90 for the rear differential.
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Old 05-02-2017, 09:51 PM   #6
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Alright, thanks for the input! the 75w140 is pretty expensive anyway, but I figured I would see if maybe for whatever reason, it makes some kind of difference. I am just daily driving the car, maybe do some track days or something later. PA90 it is!
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Old 05-06-2017, 01:40 AM   #7
grey wolf
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So here is some food for thought. Everyone seems to recommend the Motul 90PA for the rear diff on LSD Subarus, but I question that logic. I understand why nobody would (typically) use the competition 75w140 given the cost, and it is quite a thick oil. But let's compare the 90PA with Motul gear 300 LS 75w90. To clarify, this is the gear 300 with a limited slip additive, which some of us would use in the 6 speed manual transmission in the STI cars with clutched front diffs. It is also a 100% synthetic ester based oil. The 90PA is not.

If you look at the specifications for each oil on the Motul website, you will see that the LS 75W90 has a higher viscosity index (90PA = 102, versus the LS = 161). Basically, a higher viscosity index indicates a smaller decrease in "kinematic viscosity" (which is a rating typically expressed in mm^2/s as measured by special viscosity testing equipment) with temperature increase. Essentially, as I understand it, what this means is that the LS 75w90 should actually be a better lubricant at higher temperatures than the 90PA. Not only that, but being a synthetic oil, it also will not break down from heat as badly as a non synthetic oil, giving it a longer lifespan. And of course, given that this is a multi grade oil with a cold temperature rating of 75, it will flow better at lower temperatures as well. This is good news for those of us living in colder climates.

So, if you're still reading this, I see no reason to choose the 90PA over the gear 300 LS 75W90. It is only marginally more expensive, but from the research I have done, it seems to be a superior oil. Also, if it works fine for clutched front differentials, why wouldn't it work also in clutched rear differentials?

I ended up putting this oil in my rear diff today instead of the 90PA. My Motul distributor also recommended it over the 90PA. I'll be sure to add another reply to this thread later on if I do in fact have problems using this oil, but I suspect that I will not.

Last edited by grey wolf; 05-06-2017 at 01:42 AM. Reason: grammatical error
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Old 08-31-2022, 10:03 AM   #8
Felman
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Quote:
Originally Posted by grey wolf View Post
So here is some food for thought. Everyone seems to recommend the Motul 90PA for the rear diff on LSD Subarus, but I question that logic. I understand why nobody would (typically) use the competition 75w140 given the cost, and it is quite a thick oil. But let's compare the 90PA with Motul gear 300 LS 75w90. To clarify, this is the gear 300 with a limited slip additive, which some of us would use in the 6 speed manual transmission in the STI cars with clutched front diffs. It is also a 100% synthetic ester based oil. The 90PA is not.

If you look at the specifications for each oil on the Motul website, you will see that the LS 75W90 has a higher viscosity index (90PA = 102, versus the LS = 161). Basically, a higher viscosity index indicates a smaller decrease in "kinematic viscosity" (which is a rating typically expressed in mm^2/s as measured by special viscosity testing equipment) with temperature increase. Essentially, as I understand it, what this means is that the LS 75w90 should actually be a better lubricant at higher temperatures than the 90PA. Not only that, but being a synthetic oil, it also will not break down from heat as badly as a non synthetic oil, giving it a longer lifespan. And of course, given that this is a multi grade oil with a cold temperature rating of 75, it will flow better at lower temperatures as well. This is good news for those of us living in colder climates.

So, if you're still reading this, I see no reason to choose the 90PA over the gear 300 LS 75W90. It is only marginally more expensive, but from the research I have done, it seems to be a superior oil. Also, if it works fine for clutched front differentials, why wouldn't it work also in clutched rear differentials?

I ended up putting this oil in my rear diff today instead of the 90PA. My Motul distributor also recommended it over the 90PA. I'll be sure to add another reply to this thread later on if I do in fact have problems using this oil, but I suspect that I will not.
And now there its me at this point .... hesitating between Gear 300LS 75w90 for my STI "06 rear diff .... or the 90PA ...

5 years later, anything to say greywolf?
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Old 08-31-2022, 10:34 AM   #9
xtremewlr
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Quote:
Originally Posted by Felman View Post
And now there its me at this point .... hesitating between Gear 300LS 75w90 for my STI "06 rear diff .... or the 90PA ...

5 years later, anything to say greywolf?
I use Redline 75w90 GL-5 in my 06 STI.
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Old 08-31-2022, 11:18 AM   #10
Felman
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Originally Posted by xtremewlr View Post
I use Redline 75w90 GL-5 in my 06 STI.
yea but, for the tranny? ... I was asking for the rear differential, that is LSD clutch/plated type ... you know, needs the LS aditives .... so wondering which better, if the mineral 90PA or the new synth Gear300 LS 75w90 (the version with LS additives)

:P
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Old 08-31-2022, 12:24 PM   #11
xtremewlr
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Quote:
Originally Posted by Felman View Post
yea but, for the tranny? ... I was asking for the rear differential, that is LSD clutch/plated type ... you know, needs the LS aditives .... so wondering which better, if the mineral 90PA or the new synth Gear300 LS 75w90 (the version with LS additives)

:P
Redline 75W90 GL-5 is for the rear diff. The Redline oil doesn't require additives for the LSD clutches.

Quote:
Contains additional friction modifiers for suitability with clutch-type limited slip differentials - for most LSDs, no additional friction modifiers are required
https://www.redlineoil.com/75w90-gl-5-gear-oil

Redline 75W90NS GL-5 gear oil is for the tranny.

https://www.redlineoil.com/75w90-ns-gl-5-gear-oil

If you do some further research, some people recommend a combo of the 75W90NS and a single quart of the Lightweight Shockproof for the older transmissions such as ours, which have the internal oil pump, to help with shifting smoothness. Do a search on the forums for "cocktail". I think maybe @unabomber has covered this as well. The newer transmissions will use 2 quarts or even all shockproof in the trans since they do not have the internal oil pumps.

https://www.redlineoil.com/lightweight-shockproof
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Old 08-31-2022, 12:50 PM   #12
Felman
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Yeah, I know all that, and all the cocktails ... there are some of them, the thing is, I am from Europe ... and Redline its great but, it's american stuff ... and here is quite difficult to get it over here,,, instead of Motul, that we have plenty of it here, every single shop has it and its "very" cheap ... that's the reason I wanna stick with Motul, and that's why was asking greywolf
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Old 08-31-2022, 03:45 PM   #13
xtremewlr
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Quote:
Originally Posted by Felman View Post
Yeah, I know all that, and all the cocktails ... there are some of them, the thing is, I am from Europe ... and Redline its great but, it's american stuff ... and here is quite difficult to get it over here,,, instead of Motul, that we have plenty of it here, every single shop has it and its "very" cheap ... that's the reason I wanna stick with Motul, and that's why was asking greywolf
I'd run the 300LS for the rear differential if I was running Motul stuff, per greywolf's recommendation at the end of his post. Likely he is no longer active on the forums, so you might not get a response from him.

If that oil is no longer available, then run the 90PA.
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