56 Rear End Fluid Change

[Wooly15]

Per the manual it says to use "Hypoid Gear Lubricant" or SAE 90 gear lube if not available. What is hypoid gear lube? Has anyone ever had negative results with SAE 90? My rear diff has developed a leak, so I guess that's my next project.

[NTX5467]

"Hypoid Gear Lubricant" was probably the stated name of GM's SAE 90 rear axle lubricant.  The type of rear axle is "hypoid" in design.  The GM lube probably came in metal cans of 1qt amount.  "SAE 90" is normal rear axle lube which could be sourced from many places other than GM dealerships.

 

To be fair, GM would claim that HGL was formulated to GM specifications, which it normally would be as it was in GM packaging, whereas SAE 90 from other sources might not be quite the same . . . or could do the "Meet or Exceed Factory Specs" orientation.  There were probably some SAE 90 oils that were not designed for rear axle use, so getting "the factory good stuff" could be important..  Every service station I knew of back then had a roll-around barrel of SAE 90 rear axle lube that was used in most all vehicles.  Manual transmissions used it too.  In more modern times, almost every SAE 90 lubricant I've seen is "rear axle lubricant". 

 

NTX5467
 

 

[Wooly15]

So what you're saying is, any 90 gear lube will work? I don't have to find special HGL?

[JohnD1956]

Yes,  any 90 wt gear oil will be sufficient.

[Beemon]

For Conventional, Castrol 80W-90 is the best off the shelf. For Synthetic, Mobil1 80W-90 is the best. I used Synthetic when I swapped rears and haven't had a leak yet. Just make sure to use some black RTV on the gasket flanges when you dig into it. Are you leaking from the axle flange or the differential gasket?

[Wooly15]

1 hour ago, Beemon said:

For Conventional, Castrol 80W-90 is the best off the shelf. For Synthetic, Mobil1 80W-90 is the best. I used Synthetic when I swapped rears and haven't had a leak yet. Just make sure to use some black RTV on the gasket flanges when you dig into it. Are you leaking from the axle flange or the differential gasket?

I'll probably stick with conventional. It's leaking from the diff gasket.

[avgwarhawk]

Hold your nose when you crack that pupply open.  That old gear old stinks the high heavens. 

[Daves1940Buick56S]

I know there are lots of opinions on this, but if it were mine I would use GL-4 grade lube like StaLube 85W90 from NAPA rather than a GL-1. This gives you the EP protection that the hypoid gears need but is not harmful to yellow metals. Since the only yellow metals we have in our (later) rears is probably the brass thrust washers, you could probably use a modern GL-5 as well. But do not put GL-5 into the transmission, it will eventually ruin your syncros!

 

Cheers, Dave