Fresh rebuild of my 68 differential

[OregonNed]

Yesterday I picked up my truck a second time after a complete rear end rebuild that started about a month ago. The first time they installed Sierra ring and pinion gears with a new Eaton Posi carrier - 3:42 ratio. The differential whined and howled pretty severely, so I had to take it back in. They either tried another set of the same gears, or installed another new set, and still had the same issue. Finally after the 4th of July weekend they installed a new Yukon ring and pinion - much better. I only hear a very, very minor amount of sound around 50ish, I am sure attributed to break in. So far I have ran 2 moderate 20 mile drives with complete cool-downs of the differential - and now in a few hours overnight I will be taking it around 70 miles (all freeway) to a big truck show. Per the break in instructions I will be maintaining around 60ish MPG for the next phase of 100 miles.

My real question is what oil to use after the 500 mile change, and what the best might be. It was finished off with Valvoline synthetic 75W-90 and additive added - however, I have researched conventional 80W-90 with additive is recommended by Eaton (GL-5 rated). Does anyone have any good advice on which oil to use when I make that change? Thanks in advance!

[jessemthompson]

There shouldn’t be any hum from the rear end period, break in or not. It should be quiet. If it’s not, it will lead to a failure. I had new gears put in a mustang years ago by a shop. It had a slight hum to it. 3 months later I’m in the side of the road with a pinion bearing failure. This past weekend I put an Eaton posi in the rear of my 72. I cannot hear anything, just like it was stock. If you have a noise I’d take it back.

[Grumpy old man]

Here's a good gear oil comparison from 5speeds.com starts at 11:00 . I'm using the Driven brand with additive in my posi 3.73 and the Muncie M-22 4 speed .

https://www.youtube.com/watch?v=Z7NpsMZ9pBQ

https://www.youtube.com/watch?v=9-yG3D3JBRs

[68Gold/white]

Originally Posted by jessemthompson

There shouldn’t be any hum from the tear end period, break in or not. It should be quiet. If it’s not, it will lead to a failure. I had new gears put in a mustang years ago by a shop. It had a slight hum to it. 3 months later I’m in the side of the road with a pinion bearing failure. This past weekend I put an Eaton posi in the rear of my 72. I cannot hear anything, just like it was stock. If you have a noise I’d take it back.

YES!!!^^^^^^this EXACTLY!!!

Synthetic gear oil is generally NOT recommended for posi rear ends. It can be used in lock up rear ends.
I'd use 80W90, non synthetic and the GM additive in a posi rear end.

[LONGHAIR]

I haven't done it for years, but things like this don't change. I worked in a 4 wheel drive truck accessory store through the 80s and into the early 90s. In that time I did hundreds of gear swaps. This was mostly because it was the beginning/heyday of lift kits and huge tires. The biggest percentage of these were gear ratio changes to compensate for that. Traction devices (posi, locker, tru-trac, etc) were added to many, especially the rear, because a carrier swap was required for the ratio jump.
All that to say, jessemthompson is correct. It should not make noise at all, ever. Noise is a function of a bad fit. It will not "wear in", it will destroy itself. The teeth wear and put particles into the oil, which kills bearings. The contact of hypoid gears is supposed to be so smooth/fluid that you never notice it.
This does not however diminish the "break-in" period/procedure. It is still very important, but is not a cover up for a bad install.
The lower the ratio (higher numbers) the more important it becomes. It's all about heat cycles and that heat's effect on the gears. Low ratio gears spin a lot faster than the higher ones that came in those emissions era (80s)trucks especially. Shorter and more frequent heat/cool cycles are better there.
Your 3.42-1 doesn't really fall into that though, usually 4 and above would get that.
Back then we used Amsoil synthetic on anything that it was approved for, not all were. The 3.42s should be fine with pretty much any good quality 75w90 and the correct friction modifier/additive. If I was doing it today, especially on the initial run, before the first change, I would use a 90w-145 and maybe switch to a lighter oil before winter, of you're in a cold area of the country.
In essence, take it back and make sure that they are aware of it and that you are not absolving them of the responsibility on this. It may live a while, but lasting long enough to get past some 90 warranty is not acceptable. When done correctly, they should last nearly indefinitely.

[OregonNed]

Thanks all for the thoughtful answers. today's 140 miles on the freeway gave me the chance to listen to the truck overall at various speeds, and with the window open during part of that. I strained my ears listening and maybe I am just being paranoid with what I think I heard. I believe that if anything now it may be the higher revs now with the engine with the gear change. Though set up properly before, there was a huge howl at 50 MPH plus, and started to get pretty bad between 25-30 MPH. Today the truck ran smoother than ever at the higher speeds, and a lot of that is also due the the driveshaft rebalance that opened this whole can of worms. It was very quiet at highway speeds and I could not detect any noise from the rear - just engine and exhaust noise. I also notice off the freeway when my truck downs shift and slows to a stop, (TH 350) that there is an audible slight whine in the front which each shift - but that has always been there. For now I think I was a little paranoid, and probably cried wolf. There is definitely no problems like before with howl and whine and sounding like it would fall apart.

I really appreciate the advice on the oil choice. I will go that conventional oil route I think.

[Jiggers72-GMC]

Glad you gave it a good run today, it 'sounds' like the differential is ok

and I also agree with @68Gold/white
''I'd use 80W90, non synthetic and the GM additive in a posi rear end''

[tim_mc]

Ned, I went through the same thing a few days ago with my own 3.42 posi upgrade with Yukon parts. The whine from the old set up is definitely gone and it's MUCH quieter in the cab, most notable on the highway. I think I also got rid of some related vibration (drive shaft was previously cut and balanced when I swapped in my 700R4). I try to keep an ear open for any sounds - so far so good. It's a bit different now that the shift points have changed. Here's hoping both of our differential builds outlast us!

[OregonNed]

Originally Posted by tim_mc

Ned, I went through the same thing a few days ago with my own 3.42 posi upgrade with Yukon parts. The whine from the old set up is definitely gone and it's MUCH quieter in the cab, most notable on the highway. I think I also got rid of some related vibration (drive shaft was previously cut and balanced when I swapped in my 700R4). I try to keep an ear open for any sounds - so far so good. It's a bit different now that the shift points have changed. Here's hoping both of our differential builds outlast us!

Thank you! agreed!