# 1970 GTO; What is best Posi Rear gear lube?



## TheBestVMan (Apr 19, 2010)

Hi All,

I have a 1970 GTO and I just changed the oil in my rear axle 3:55 posi limit diff. The problem is now the rear slips pretty badly in each gear. I used Mobil 1 75 -90W synthetic gear oil and GM posi additive, which were recommended by parts store. My mistake. The rear ran fine before I changed the oil so I'm going to change the oil again and hope that fixes the problem.

After reading other posts, my question is which rear oil should I use? And does it slip because I used synthetic or because I used 90 and not 140?

The most common answer I've read is to use either Torco rgo 75w 140 or Torco rgo 85w 140.

Any thoughts would be appreciated.


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## rickm (Feb 8, 2012)

TheBestVMan said:


> Hi All,
> 
> I have a 1970 GTO and I just changed the oil in my rear axle 3:55 posi limit diff. The problem is now the rear slips pretty badly in each gear. I used Mobil 1 75 -90W synthetic gear oil and GM posi additive, which were recommended by parts store. My mistake. The rear ran fine before I changed the oil so I'm going to change the oil again and hope that fixes the problem.
> 
> ...


lucas heavy duty high performance sae 80w-90 gear oil with 4oz. bottle of ac delco limited slip axle additive. that's the ticket. rickm


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## geeteeohguy (Feb 2, 2008)

I agree with rickm. Any heavy duty NON synthetic oil with the additive, and in 80w-90.


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