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Manual Transmission Oil 75W-90 vs 75W-140 for a Gen1 987

rick987s

rick987s

PCGB Member
Member
Hi guys, hopefully someone can resolve this mystery regarding Manual Transmission Oil for a Gen1 987 because I've been reading different things in online forums.

Does it matter whether you put 75W-90 or 75W-140 in the Manual Transmission of a Gen1 987? My trusted Indie Specialist recently put 75W-140 in my transmission when doing the 12-year fluid change because they said it exceeds Porsche's spec & therefore would be better long term & the higher viscosity would offer better protection when hot but I'm not convinced.

I've looked carefully through the owners manual & other than stating that you should use Porsche's own fluid, there's no mention of a make or viscosity or any other spec.

I therefore spoke to my trusted local OPC parts guy who checked their oil for any info as apparently, it only gets supplied to OPCs in 20 litre containers.

The OPC parts guy confirmed that the only thing written on the barrel was that it was Porsche Hypoid Transmission Oil & nothing else.

Therefore, where is it written that Mobil1 is recommend for the transmission & what the viscosity should be?

Thanks for any help/advice.
 
This
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I can just about see the part number but my OPC gave me this part number: 00004330500

That looks different to me plus they informed me that it doesn't come in 1 litre containers; you have to bring your own.
 
Rick,

There was some forum chat about this fairly recently if you want to search for it, but I'd have a word with Opie Oils https://www.opieoils.co.uk/

They can recommend an alternative because, as you say, the Porsche oil isn't available in small quantities. Also, they give a discount to PCGB members.

Jeff
 
Hi Jeff.

Thanks for your reply. I already tried searching using the word "transmission" but it didn't yield anything for me. As an admin on another forum, I appreciate how annoying repeated questions are but perhaps I was seacrhing using too broad a term or in the wrong group?

Anyway, thank you for the link to Opie Oils as that's very useful but if you could point me in the direction of the said post, I'd really appreciate it.

That's why I always try to insert an exact & correct topic & tag my posts; to make them easily searchable for other members.

Many thanks, Rick.
 
Rick,

The only reason I didn't include a link to the post was that I couldn't find it myself.! There was one similar to yours posted in February https://www.porscheclubgb.com/forum/tm.aspx?m=1019403 but I note that I gave the same advice to the OP as I gave you to contact Opie Oils.

There's a chap on the forum (oilman) who I think works for Opie. You could PM him for advice but you're probably best contacting them directly.

I suppose you could always see if your local Porsche Centre will sell you some oil if you supply a suitable receptacle.

I have to say that I'm sure that the oil your specialist used will be perfectly OK given that it meets - or exceeds - Porsche's spec and that it has a wider viscosity range.

Jeff
 
The correct viscosity is a 75w-90, I believe the factory fill fluid was called Castrol TAF21 which is just an internal Castrol reference. Moving to a 75w-140 will not gain anything, it is being used on the basis that thicker is better but this just not the case.

To give you an idea the average 75w-90 is around 14.1 cst (centistokes) at 100degc and an average 75w-140 is around 24.5 cst at 100degc so nearly twice as thick at operating temp. What is this extra viscosity doing for the gearbox? Well not a lot is the short answer, in a road car it will not be adding any more protection to the gearbox over a 75w-90 that meets the correct spec. All it is doing is causing extra drag within the gearbox, sapping power and causing extra heat and friction. 75w-140 does have its uses, in very big power applications that may see excessive shock loading on the gearbox and 24hr endurance racing and that's about it.

We have many quality alternatives to the standard Porsche fluid and in some cases better. The likes of the Motul Gear300 75w-90, Redline, Millers CRX NT+ and the Gulf Competition are all sat at the top of the tree because they have the advantage of containing synthetic esters. In 987 if you want to upgrade the gear oil, go better not thicker.

Cheers,

Guy
 
Many thanks for the clarification Guy. No doubt Rick will take the advice on board and, as I've suggested, get in touch with you.

Jeff
 
I used Mobil 1 75w-90 last year when I changed my gearbox oil at 95k miles and all seems to be fine. You’ll need an oil syringe if you’re doing it yourself but it’s pretty straightforward.
 
Hi everyone.

Thanks for all your advice. Guy from OPIE Oils thanks for your explanation. In the case of the indie garage I used, I suspect it was a matter of the fact that they may have bought a job lot of it so they used that in my car.

So it seems as though I'm presented with 2 situations; either pay again to have it changed which is going to cost me around £100 inc labour or I put up with it for a while & maybe change it next year again.

I'll monitor the situation to see if it affects fuel economy but so far, I can't see any difference. The car isn't my daily driver & I don't seem to do more than about 3500 miles a year in it so as long as its not going to be detrimental, I think I might leave it alone for now & review the situation in say 6 months time.

Unless you all think that's ill advised or I should complain to them about the choice of oil of course.

Cheers, Rick.
 
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