I'm replacing my stock handguards with a DD Omega X FSP, and fig'd that I'd completely disassemble the upper and give it a good scrub. I've got 10k+ rounds through it and have performed routine maintenance, but thought it would be fun to take it all apart since I almost have to for the rail install.

The joint where the barrel fits into the upper calls for a durable high temp grease, and I was hoping for some suggestions, preferably something I can get at Home Depot since I need to get a breaker bar from there today anyway. I'll probably end up using the same grease under the FSP, since apparently LMT didn't feel like phosphating those two little circles.

What can y'all recommend? Thanks!

Would something like this Lucas white lithium grease do the trick? Any chance that would eat away at the finish inside the upper or on the barrel?

http://www.homedepot.com/p/Lucas-Oil-8-oz-White-Lithium-Grease-10533/202535870

Nickel Never Seize     don't overdue it, don't get any on you.

Cool, thank you!

Doesn't MIL SPEC call for Aeroshell 33? That is just a general purpose airframe grease. Amsoil Lithium should be easily available. I don't think that nickel anti seize not is appropriate. 

A lot of anti-seize compounds contain graphite.
Aeroshell 33MS meets the mil-spec.
I think it has been replaced with a different part number to prevent confusion between 33 and 33MS.

It's Aeroshell 33MS (now called Aeroshell 64), which is a lithium complex grease with synthetic base oil and molybdenum. 

Honestly, any good Li-Complex grease will do fine, even the generic red/tacky automotive stuff.  You just need something to allow the threads to slide together a bit so you can make the torque properly and help protect the threads from corrosion.