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Originally Posted by Adam
I'm checking into gear oil for the Porsche trans and I'm on Redline's site. They recommend their GL-5 75W90 oil for Porsches not the GL-4. This is contradictory to what some of you are saying. I'm confused.  Here is the link.
http://www.redlineoil.com/Products.aspx?pcid=7 |
The problem is that you would normally use a GL4 in the transmission, because as I understand it most GL5s are corrosive to the soft metals found in there (copper, brass). However, the Boxster uses the same lube in both the tranny and the differential, and the diff has hypoid gears.
Redline specifically says that their MT-90 GL4 is *not* for use with hypoid gears. They also say that their GL5 is formulated so it is not corrosive to soft metals. That's why they recommend their 75W90 GL5 for the Boxster.
Having said that, I was told by a 'reliable source" that the MT-90 was just fine to run, and I did. My shifting smoothed out, and I never had any issue with gear noise over 20k miles of use. But if I still had the car I'd swap it over to the GL5 lube, just in case.