Differential Fluid Change?

Hi all, I have a question on performing normal maintenance on my 71 Buick Skylark that I would like some input/feedback from member experience. I have checked the service manual on my Skylark regarding fluid change
on the standard differential (273 gears) and it states that fluid change and or flush is not recommended (just maintain fluid level). I have searched the forum on this issue and can't really tell if this is normal practice or not. From what I can tell my lark has 40+ year old fluid and I would think it would be beneficial to change it out and put fresh 90 Wt. oil in there. Is there a reason the factory doesn't want us to change oil? Is a fluid change going to cause problems after this amount of time or do they just want their own dealers to do this for some reason? Would appreciate some feedback and or member experience on weather this is ok for us "shade tree" mechanics to do. Looks like I would just need to remove the rear cover, drain, clean and reseal with RTV or some form of gasket (if still available)?

Thanks for your thoughts!

John

 

LOL By all means its well overdue my Skylark friend. Drain the old 40+years of fluid and refill with fresh 90wt you,ll notice the difference.

 

WOW never heard or seen anything that says you should never change the fluid. lol.

BUT in the new cars they do not have dipsticks for the trans... They do not want people in there. I have seen people put washer fluid in the over flow for the rad and vice verse... I guess people could end up putting wrong fluid in the trans etc.

With a OPEN none posi once the gears are broke in I would say you probably do not need to do the oil for a LONG time. BUT a Limited slip...??? I say that is bogus info. Clutches and cones will tear up fluid I would say at least 10-15K

I would guess it is your call.

IE I have a riding mower from Craftsman. I HAVE never changed the oil in. EVER...??? 4-5 year old now. Runs like a top. all I went through was a Battery. Never did a belt (which by the way I will be getting next week. It is as thin as a hair...
I am no say this is right by any means and DO NOT condone doing it. I think it is terrible, but I have already done it. After seeing my neighbor totally change the oil and filter crazy frequently on his riding mower (craftsman) and it BLEW up the motor sending the rod though the side of the block...??? WHO KNOWS what is right anymore....

CHANGE THAT OIL!

 

Thanks for the feedback; I checked to see if it was a posi unit and it looks to be just a standard gear set. My Skylark has about 75k miles on it and there are no current problems with noise or leaks.
I purchased a felpro gasket from Napa today and should be good to go for a fluid change.
It looks like the most fun will be cleaning up the mess and finding a way to pump the fresh oil into the fill port on the differential.
I appreciate your thoughts as I didn't really want to damage the carrier bearings or seals with fresh oil (sounds counter intuitive I know) , it doesn't sound like there is an issue!

John

 

the old oil will stink like crazy. When I had the rear gear oil changed back in 2000 when I purchased my GS, the mechanic had to flush the pot and axle tubes.

 

Yup, that's what the service manual says, just maintain fluid level.

Funny enough, when I worked at a Pontiac/GMC dealership 15 years ago, one of the mechanics there was selling a '69 Lemans he had bought new, and for what ever reason the subject came up on the differential had never been opened and he said, "Yeah, you don't want to open it now, it has that good whale oil still in it." At the time, I wasn't sure if he was joking or not p

 

thought whale oil was used only for the posi differentials. Maybe the lemans had a posi.

 

I know FORD was still stating this in the 90's for their pickup trucks! Service manual recommendations were to never change the fluid unless the axle was taken apart for service or was ever submerged in water.

I have changed gear oil in a 50's era truck (axle and trans), and I can say it wasn't pretty. Think chocolate pudding.

 

To change or not to change; sounds like I am getting "wrapped around the axle" here So I may be taking a small chance in changing but I guess what most of you are saying is that it is worth the risk!?
So I have some gear oil at hand both Sta-lube and some Ams Oil 75-90 wt. Should I just stick with the standard fluid or go with the Ams oil? sorry, not wanting to take this post in another direction but would
think now that I have peaked some interest with the question, will get some direction. Thanks for your contribution to my solution.

John

 

Back in the 1980's, an old drag racer told me he rebuilt his Chevelle 12 bolt posi rear end every winter.
Then he said... "three years ago I switched to synthetic fluid and it's still going"!
Ams Oil should be an excellent choice.

 

My eaton posi unit I purchased from Monzaz had a similar warning message..

 

As did mine. I used Brad Penn gear lube and a bottle of the GM additive.

 

Thanks for the input on Posi fluid; I'm assuming though that since mine is a standard (non posi) gear set that using synthetic would not be an compatibility issue? I'm more concerned that if I change to synthetic after 40+ years of
using conventional gear lubricant that it may cause more harm than good. I MAY be overthinking this as the old conventional oil in there is certainly contaminated with gunk at this point. Is using synthetic similar to using a detergent
motor oil after using non detergent? I'll try and clean up the case and surrounding area with rags when I take the cover off but will not be able to "flush" the tubes or gear set for a clean install.

I do appreciate all the feedback and opinions... definitely helpful in making the right decision.

John

 

NEVER use synthetics with any clutch style posi unit.

 

The only caution I would have against using synthetic in your open rear would be how it interacts with the seals. It could cause a leak you may not have had before. This could certainly be misinformation but it's something to think about unless someone mentions otherwise.

I don't see a huge benefit to going synthetic. You might cut down on some heat and parasitic loss but I'm sure it's not a perceptible difference.

If you set a fluid change interval and stick to it, your changes in the future will be quick and easy. No gunk to clean up. You could also look into a diff cover with a drain and fill plug in it to make things quicker.

 

Ford does require synthetic in the 8.8 units and they are clutch...BUT they also are Composite clutches (glued on material)

All this is crazy. YOU can not go wrong putting new oil into ANY mechanical item. New Oil is ALWAYS better than old 40 year oil PERIOD.

Posi units back in the day when we had only STEEL to STEEL components like Auburn and Eaton both used steel to steel - when these components wear in they get sooth and create a very large smooth surface area and shine up creating a kind of suction adhere affect. This cause the clutch and cones to not release smoothly. They would BUST loose and cause that chatter affect you get. So going around the corner slow would make a violent slip grip, slip grip and in the car it would feel like banging and spider gears would clank and chatter etc. So the additive kind of makes the oil less conducive to creating the foam and air pockets that would lessen the suction affect.

Now the Eaton a unit are Carbon Fiber and composite and the need for the additive is much less.

So if you are running Steel clutches or Auburn cone RUN THE ADDITIVE - and conventional gear oil NON SYNTHETIC
If you are running Composite or Carbon fiber - you can try with or with out the additive to see what affect it might have

Lots of nostalgic racers do not run the additive for better hook and deal with the clutch and cone issues A trade off so to speak.

Jim
J D

 

Thanks to you all for giving feedback on this question; as you recall this started out with questioning the service manual write up recommending not to change differential fluid after break in. It seems that
the practice of Buick owners is to change the fluid anyway and the consensus is to use conventional spec gear oil fluid on both posi and non-posi units with additive (fully answering my questions). I'm hoping others
that question what is normal will now have forum opinion on what works and what is of concern.

As always members on this site take time to share what they know works so that the rest of us can all learn from collective experiences!

Happy Buick motoring!
John

 

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