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Aeroshell33 meets the “mil-spec” IAW the -23. But the lithium grease you’re using should work just fine. You simply want to prevent galling of the threads during assembly.
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I’ve built 6 or 7 lowers now, and never used grease on the extension tube threads before. Did i miss an important step? where are you putting it: just under the castle nut, in the reciever threads, both? what is the purpose? I know you said “to keep the threads from getting galled”, but i’m not sure i’ve ever experienced that without the grease. Is it common? I’m curious, this is the first time i’ve read about that.
I use Loc-Tite C5-A anti-seize. The reason for it is because the steel and aluminum sometimes have a tendency to “fuse” together over time and depending on various factors. It is much easier to remove a castle nut that has had a small amount applied to the thread area than one that hasn’t.
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good info. i’ll try it on my next project.
Follow the link below to the “Basic AR15 Cleaning” tacked thread. Scroll down to just above where my photos begin. There you will find a link to the -23&P. In the -23&P under the R&R of the receiver extension, you will find the call-out for grease. 
If you re-read IGs post, you see the Loc-tite he mentions is C5-A. This is Anti-Seize paste made by Loc-tite. It is not Loc-tite thread locker. C5-A is used extensively in the aviation industry to prevent the corroding of fastener threads.
Google Loc-tite C5-A for more info. 
You’re supposed to tighten the castle nut to forty inch pounds. How do you know when it is this tight?
Typically by using a torque wrench, though I never have and have never had one come loose.
I’m not useing a torque wrench either, but about how many ft-lbs does it take to equal forty in-lbs?
Skip
Yes.
C4
Couldn’t tell you. But, maybe you can Google how to convert ft./lbs- in./lbs?
http://www.jaec.info/Measurement/Torque-Converter-Measurement/Torque-Measurement-Conversion.php
The simple answer is 1 ft lb = 12 In lbs.
A ft lb torque wrench will generally not be accurate enough to properly measure 40 in lbs, although castle nut torque is not a critical measurement (within reason).
Thanks.
I use Hornady One Shot Cleaner Lubricant on the extension when installing it. That way, there is no residue to collect dirt after assembly. Works really well.
Residue shouldn’t be a problem after assembly. The grease is applied to the threads of the receiver extension and lower receiver only. A small amount to prevent the possibility of galling is all that is required. Any more grease than this would be overkill.
ETA I:
ETA II:
Resource for grease: http://www.skygeek.com/5067068.html