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transmission fluid help

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I've searched a bit on different forums and collected some information, but still not sure what to use. It seems the dealers buy in larger quantities and don't always use Mobil 1 or other approved oils. I doubt they carry separate types for the older 2.5l boxster compared to all other Porsches, so they put the same thing in all Porsches. That means many of you guys with 2.5l boxsters might have the wrong fluid in it.

Now the two ratings are GL5 and GL4, each one is different due to the alloys used in the tranny. Some have friction modifiers or slippery additives and some don't. (mainly for limited slip)

The 2.5l boxster has a 5 spd that's also used in Audi's. It requires GL4 rated transmission fluid which was made by Burmah and got bought out by BP. Mobil One does not make GL4 fluid.

Most other Porsches like the 996 take a GL5 rated fluid:

Shell Transaxle (75W-90), 999.917.546.00 for 1 liter bottle

Mobil Mobilube PTX (75W-90), though I don't know what PTX is compared to their standard synthetic stuff.

Other non approved fluids that people have used:

Redline MT-90

Redline 75W/90 NS (2 parts) mixed Redline Lightweight Gear Oil (1 part)

Royal Purple (claims to be compatible with GL4 or GL5) has friction modifiers in it

Pensoil for GM cars

Does anyone have more information they can add or suggestions? Thanks,

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97-99 manual tranny was almost idenical to the Audi A4's.

But the factory LSD wasn't availble, however GL-4 oils are for transmissions that contain brass or bronze parts.

GL5 oils differ from the GL4 oils in the EP additive. The most known EP additive is a phosphorous-sulfur compound that is corrosive to brass at the concentration used in a GL-5 oil,,,

There are GL4 ratings out there like Pentosin ATF we used on a few older jettas & Amsoil Synthetic API GL4 75W-90 would be my choice.


Edited by juniinc
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(My car is a 1999 Boxster 5-speed)

On my first transmission fluid change (60k miles - earlier than scheduled), I did some research and decided to go with RedLine 75W-90NS. Afterward, shifting seemed slightly improved over the worn stock fluid but over time it definitely felt more notchy.

At 115k miles, I changed the fluid again but this time back to stock Porsche fluid. Shifting was noticeably improved. So I don't know what they are using but I like it.

I spoke to RedLine about making a cocktail and they were very sure that this was neither recommended nor necessary. They also said that 75W-90NS was the only of their fluids that was appropriate for the Boxster transmission.

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