Selection of the Right Transmission Oil for the Corvair and other Transaxles

Click here do download the pdf file


I constantly get asked or read questions on forums where people are confused about the API gear oil categories and the responses they get from oil company "technical" people who apparently answer the wrong question. With this in mind and the specific need for a transmission oil that does not harm synchronizers but yet can protect the complete transaxle in cars like my Corvair, I set out to explain the variables and the spec sheets.


I have set out to explain the fallacies of these myths:

  • GL-5 oils are better than GL-4 oils and that the GL-5 category replaces the GL-4.
  • GL-4 is the same as LSD (limited slip)
  • The buffered additives in the GL-5 do not harm brass synchronizers
  • Oils that meet GL-4/GL-5 can be used in synchronized transmissions
  • You need GL-5 in a transaxle because the differential has high torque
  • You can use the same oils in classic cars as specified for modern manual transmissions

I will also touch on viscosity and the problems of too high and too low.

This is the current draft of this paper.  I’m sure there will be additional questions that will arise or points that I haven’t explained to the satisfaction of some readers.  Please send your questions or suggestions to and I will answer them in the next draft.

The comments on oils in the US market are current as of November 2009. 

While I do not plan on frequent updates to this paper, as I add something I will note the major changes here. 
  • Explanation of oils for modern manual transmissions.
  • Latest developments in oils for manual transmissions

Click here do download the pdf file.                             Current update is January 7, 2016

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