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Discussion Starter #1 (Edited)
Everything I had read led me to understand there is no drain plug on the M20 in a 68 GTO, the old oil has to be pumped out through the fill plug on the side...HOWEVER! My 68 Pontiac Service Manual tells me on page 7D-7 to "Remove drain plug at bottom of transmission and drain lubricant"...so, this led me to examine the "bottom of transmission" in search of this drain plug. I do not see one, so I expect maybe some of the Muncie's had the drain plug and some didn't? Maybe I am not looking in the correct place? I have not yet been able to budge the fill plug so won't be draining anything (either through the drain plug or the fill plug) until this issue is solved. Should I be seeing a drain plug? Thanks as always for guidance.
 

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Discussion Starter #3

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Thanks Jim! I used GL4 Sta-Lube gear oil and had to buy a gallon as that is all my local NAPA store had. If I decide to change out the differential using the same oil, should I be considering a anti slip additive for my posi rear? Again, thanks for the guidance.
As far as I know, you want to put in the anti-slip additive for posi-traction rear ends if it is the factory type with the clutches. I would do a web search to make sure you get the correct type for the older car. Everything has changed with the new cars, so sometimes the "new" stuff is not what you want.
 
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