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Rear Differential fluid


56 Rum Runner
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I'd like to replace the fluid in my rear differential. I know there isn't a drain plug because Buick never intended for it to be replaced, but I think it looks contaminated so I would rather spend a few dollar on preventative maintenance than allow it to cause more costly problems. My local mechanic has a tool that will extract the fluid through the fill hole and and then refill it. What weight oil does everyone recommend (80w90, 95w 145). Do you just put in the straight oil or are there any additives that need to go in as well? The car is a 56 Special by the way. Thanks. Mike

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Hey there might, There should be a drain plug right in the middle at the bottom of the back of the diff. (thats where mine is) Anyway, just drain her out and make to clean it up real good, new gasket and straight 90w is what you want. Take about half a gallon. I just did this on mine last week. That old original 90w was real thin and it was leaking out the bottom of the diff. Everything is great now. Napa sells straight 90w for about 12 bucks a gallon.

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I hope someone with a 56 jumps in here. The rear in a 56 changed from 55 and there is no cover. I don't know how to drain, so you may have to extract from the fill hole. I find that 85W-140 gear lube is the most suitable for my 55's and should work in your 56 in all weather.

Willie

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I used a vacuum pump to suck the old oil out. Refilled to the level of the fill plug with 85w90 oil. I have used that oil in other cars so I hope is is fine in the Buick. I don't have many miles with new oil change but everything sounded fine with it.

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My brother in law is a veteran mechanic and suggested using 80w90 so that's what I'm going with. Thanks to everyone for your suggestions.

I checked and 80w90 is what I've been topping off my '59 buick differential with. I may need to go with something heavier though...wonder if it would reduce the leaking at the torque tube??? ( I know, fix the torque tube leak....all in good time)

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