Replacing Differential Gear Oil

[5TID]

I am possibly looking to replace the gear oil in my M923A2 (rebuilt in 2011, about 5500 miles on it now). I am almost positive the oil has not been changed since it was rebuilt. The oil itself looks normal, and even pretty new still, but I figure there is a time aspect as well versus just debris and discoloring.

Looking through the spec, it looks like it takes 12qts per axle (roughly 9 gallons total) of MIL-L-2105 (looks like a API GL4 equivalent?). However, that spec is deprecated and looks like it has ultimately replaced with SAE-J2360 (via MIL-PRF-2105E) spec (basically API GL5, plus some in-field testing).

Shopping around locally, I can find API GL5 oil easily (self proclaimed GL5 since there is not a third party testing requirement for this spec) from tractor supply stores (even Amazon), and pretty reasonably priced; so ideally that would be the route I went.

So my conundrum: do I spend significantly more on a true SAE-J2360 spec gear oil, possibly for more protection than I really need (like full synthetic for my 8.3 when I change oil frequently enough that it is a waste), or would the GL5 oil be more than enough protection since it exceeds the original spec oil for this truck?

While yes, I would like to save money, I am not cheap. I would gladly pay if it is required, but do not want to be throwing money away on overkill. Would be nice to hear what the group thinks, or has done for their gear oil. Thanks in advance.

 

[juanprado]

I used GL5 80w90 gear oil in my m923 and it is better in my mind than the original lube. GL5 is pretty much the standard now a days in the civy world. Synthetic is not worth the hype or cost in this application IMHO.
Ymmv

 

[Mullaney]

I am possibly looking to replace the gear oil in my M923A2 (rebuilt in 2011, about 5500 miles on it now). I am almost positive the oil has not been changed since it was rebuilt. The oil itself looks normal, and even pretty new still, but I figure there is a time aspect as well versus just debris and discoloring.

Looking through the spec, it looks like it takes 12qts per axle (roughly 9 gallons total) of MIL-L-2105 (looks like a API GL4 equivalent?). However, that spec is deprecated and looks like it has ultimately replaced with SAE-J2360 (via MIL-PRF-2105E) spec (basically API GL5, plus some in-field testing).

Shopping around locally, I can find API GL5 oil easily (self proclaimed GL5 since there is not a third party testing requirement for this spec) from tractor supply stores (even Amazon), and pretty reasonably priced; so ideally that would be the route I went.

So my conundrum: do I spend significantly more on a true SAE-J2360 spec gear oil, possibly for more protection than I really need (like full synthetic for my 8.3 when I change oil frequently enough that it is a waste), or would the GL5 oil be more than enough protection since it exceeds the original spec oil for this truck?

While yes, I would like to save money, I am not cheap. I would gladly pay if it is required, but do not want to be throwing money away on overkill. Would be nice to hear what the group thinks, or has done for their gear oil. Thanks in advance.

.
I have to agree with @juanprado .
Definitely nothing to be gained by going to Synthetic.

In my opinion, if you aren't running through creeks and mud holes you could easily get to double your current mileage and still be clean.

 

[Superthermal]

I did 9 gallons of 75w-90 Lucas Synthetic. (and the T-case and Transmission too) Since I did that I haven't noticed my outer hub leak into the drum on my two previously leaky seals. I would have normally seen a tiny bit of grease sling onto my inner side of my drivers side rear tires as has been the case since I bought this truck a year ago. Since changing the oil... nada.
Odd I know.

 

[5TID]

My transmission looks to have been converted to Dexron III, but I was going to do the transfer case with the 80w90 while I am at it.