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Originally Posted by Idaho Red Rocket 3
JFP in PA. Please explain what is going on with the Red Line oil. Why will the shifting become more difficult ? I bought some RL GL-4 oil , but have not yet done the job. I've read elsewhere that GL-5 oil is bad for these transmissions. What is the correct tranny oil to use ? Please comment. Others with details, please comment too.
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Shear appears to be the major issue. Red Line, like RP and Amsoil, all seem to start off reasonably well, but the products do not stand up to shear, particularly under hot conditions. As the result, clients had begun to complain about poor shifting and unusually noisy gearboxes after very limited use; in one case, a single track day was enough. We had the lab that does our oil analysis work test both virgin and relatively short usage samples of RL, RP and Amsoil, all were found to have fallen out of grade (advertised specs), and with poor film strength properties after a couple of thousand street miles. In a couple of cases, the used oil viscosities were very low, less than half that of the starting product. RP was the worst, with RL was close behind. Basically, none of them faired very well. In each of the cars that had problems, we fully drained the gear box of the aftermarket oil, and refilled them with Porches’ OEM gear lube, and the problems completely went away.
What everyone seems to overlook is that Porsche uses a totally unique gear oil spec, it is not GL anything, it is Porsche’s. While smaller gear oil vendor/blenders like RL, RP and Amsoil like to claim that they have the perfect substitute, as the result of the unique spec Porsche set, they really don't. We have written to some of the major oil companies (Exxon Mobil, Castrol, Shell, etc.) asking about suitable products for these gearboxes’, and have uniformly been told, "Due to the unusual factory specifications, we currently do not have an appropriate product for this application, and would suggest using the OEM's product."
This topic, by-the-by, has been covered ad nauseum on multiple sites over the years (PPBB, RennTech, etc.), always with the same conclusion; Porsche’s gear oil is the way to go. From what can be determined, the OEM product is a full synthetic, possibly made by Shell (unconfirmed). But as the result of our test, we buy the OEM oil in drums, and it is the only product we use. The OEM fill is not all that expensive, and is readily available, so why mess around………..