70 GS 3:64 posi - Differential lube recommendation

Looking to change the differential fluid in a 70 GS with a 3:64 posi.

Any recommendations as to brand and weight of gear oil to use?

 

Brand, Any brand is fine - use a NON SYNTHETIC fluid hypoid gear fluid (rear end gear fluid)

We have used Walmart brand in race cars just to prove a point...lol.
DO NOT buy addtive pre mixed in the fluid...Reason being ...you do not know how much addtive % is in the oil they are selling you. Valvoline and STP a good examples of additive in the fluid. WE HAVE had issues with these companies and chatter as it seems what ever additive or the amount is NOT RIGHT!
We have used coastal, Lucas, Super tech (walmart) you can never go wrong with GM (ac delco) FORD (motorcraft) or Mopar non synthetic oil (expensive...i know) I have heard AMSoil...BUT they are full synthetic... so be careful there.
Jim

 

Additive - Lucas, yukon, auburn, again GM Mopar Ford is good.
do not buy transx in AUTOZONE...looks like it is a tooth paste tube of additive...GARAGE... soory every time this has been added it never works.

 

Which weight?

 

aNY WEIGHT TOO. 80/90 85 / 140 it is all good. none OF THAT MAKE ANY DIFFERENCE. The amount is what keeps the rear good. check it at least once a year for level. Axle bearings in some rears really on the oil from the center ...and if it is low they will fail as they only get oil from when the car turns left and right. ...If the fluid is low they will never get sufficient oil and ...poop ...failure.
Came with carrier bearings they are only getting the oil and the top of the level and when your car is running that will go down and climb up the ring to oil the pinion at the front of the casting. ... Rears do not ask for much maintenance...BUT that oil level is critical. Never let it run low.
Jim
JD Race

 

I don't like to add the additive....just makes the cones slip...in my experience.

 

Yep I tell the street guys that the racers usually do not add additives.
It is hard to please everyone in what we do. Some are tolerant others are very picking about every little click, tap, etc. lol

 

Off the cuff question if it's ok.
Back in 1979 when I bought my 66 Skylark GS it had a bad drivers side axle bearing on the 2:93 posi. I never looked or took it apart just replaced it with rear end from a 66 GTO and threw out the one from the GS

The guy was a house painter and and I just guessed he loaded the trunk with alot of weight. Would the 66 Buick style posi or even none posi lube the axles the same way?

 

?? Trying to read this... Must be my age I am missing the question in there..
ALL limited slip rears usually need additive to keep the cone and or clutch chatter down on noise.
When you drag race guys/ girls cars are so noisy it usually is not a concern...HOOK off the line is. so they will sacrifice that for the tight hook on the components instead of the smooth quiet running car.
I will say again... these are your cars and people will do what they want and what works for them. We only try to help solve problems and give the best info we can to help. Jim
JD RAce

 

I'm sorry Jim, is it my question or the reply's from the OP question that you missing?

 

Thanks...
It was 40 years ago but now I am wondering if it was because the heavy weight in the trunk. The left wheel had a thump, thump thump feel at every rotation.
The original owner I bought it from wasn't a street race type of guy and only told me he opened it up on occasion.
Should of looked at it I guess just to see.. Oh well it was ages ago and don't want to get this thread off track anymore.. Sorry everyone..

 

Jim,
Tell me what I don't apparently know about Synthetic gear lube. I just did a posi install-new gear setup for a customer on an 8.5" rear.. and I was surprised to read in their instructions that the gear manufacturer specifically prohibited Synthetic gear lube. In fact it voided the warrantee. Now I see you saying the same thing. I did a quick google search and all I found was outfits saying how great their synthetic gear lube was...

To further add to the confusion.. all of Richmond's gear oils are touted as being "Fully Synthetic". I have had great luck with their gear lube in 9" Ford setups.

So tell me... How does the origin of the base oil matter here?

I recall when Synthetic gear lube first came out, it was all the rage, everyone in the racing world was using it in new setups, and switching out existing stuff to the new fluid. Daily drivers were switching for extended temp performance, as well as longer change intervals...Especially those of us who live in cold climates, where conventional gear oil turns to mud at -20...

So what gives?
JW

 

The synthetic is more slippery and makes it more difficult for the cones or clutches to work,which will eventually cause premature wear. ALL of the clutches that come in the new units are sub-par,compared to the originals from back in the 60’s-80’s. There are also a less number of total plates because they bond that poly crap to them,which makes them thicker,and therefore less number of clutches will fit. The material is not as aggressive to operate smoother,but then needs a stiffer 600-lb or 800-lb spring pack to try and make friction. When I do a custom build,I pitch all of that in the can,install the good knurled steel clutches,tune the unit,and do NOT run any spring pack. It will work better than ever and last longer than us.

 

Yup, I could see that being an issue with friction materials in the diff case.. but the manufacturers prohibition against Synthetics was from the ring and pinion guys, not the posi case manufacturer.

That's what I found so odd..

JW

 

There is a warning paper that comes with the differentials. I wonder if the ring & pinion company believes it effects the break-in of the gears.

 

There needs to be friction for break-in. I ran synthetic once for break-in of my 8.5 spool set up and the black oxide coating of the gears was still on the gears (noisy, so I pulled cover) after "break-in". I switched to non-synthetic and checked it a while later and the gears were shiny and polished teeth. Told me that the synthetic was too slippery. I LATER switched to synthetic.
Same thing with motors....break-in on conventional then switch to synthetic.....cams/lifters need to mate, rings need to seal.
Roller cam motors maybe different but they still have rings.

 

I just made a point to put the same conventional gear oil in everything,posi,locker,spool,whatever. Can’t go wrong that way.

 

Well I guess I am late to the PARTY....lol.
They have it all correct.
Plus the company will not warranty the rear posi unit if you did the synthetic oil.
I have parted out THOUSANDS of rear ends over 30-35 years and if they had oil...ANY KIND of old nasty oil the gears were always good and run able...Of coarse if the car sits too long in one spot the half of the gear out of the oil could suffer and get flash rust and eventually heavy rust from whether changes in the north.
So always move the rear 1 once a year if you can spin it 360 forward and back wards ...SAME with a motor spray the intake with heavy amount of WD$) and hand spin the motor...I guess to could kill the ignition and crank the starter too. You do not want any parts sitting too long in one position when in storage. Valve springs valves cylinders can all suffer.

Jim
JD Race