Should the fill plug on a Dana (or any other rearend) have some sort of thread sealant applied to it?

no, not generally, as you fill just below the plug, if anything maybe some teflon paste to prevent it from rusting or leaking if so desired

I use a light circular smear of Permatex #3 around the threads with the shoe polish type brush. it is thin/goes on easy/seals well. I ain't sure if that is pipe thread or not.

The plug should have NPT, National Pipe Threads, which are self sealing due to the threads being tapered from small at the opening to larger as it threads in, just like pipes do

If you don't put some type of sealer on it, it will leak.

Agreed, mine leaked before I added some teflon tape on the threads.

You'll notice all plumbers use sealant on NPT thread joints.

Joe

That's why Carter says not to use sealant on their fuel pump threads.

Anyone that can't manage to cover their asscrack is no go to source in my book.

Now plumbers who can manage to cover their asscrack know that NPT doesn't need sealant. Burr free and a lube is all you need.

I do not use any sealant. No leaks from the plug. It is not under pressure (assuming the axle vent is installed and clear!). No harm in a little sealant but not normally used at this location in my experience.

As far as tapered pipe plugs go in general, there are different classes of thread quality. The highest quality (rarely available) does not need any any sealant in pressure applications. "Standard" quality needs sealant. Some of the stuff the home improvement store sells will not seal regardless of what sealant is used in my experience.

the theory behind a tapered thread generally, is the sealing ability without anything applied topically to complete the seal. but the quality of the taper threads, material, and the threading process, all play into this equation. finding "quality" NPT threads can be quite difficult these days, especially in hardware store plumbing supplies, as well as the typical automotive NPT threaded item. in general, most will seal, but the "seal" will be "damp" without leaking. in other words, you will see dust clinging around the fitting, or it will be damp with nothing leaking or dripping. i generally use a dab of loctite 567 teflon sealant [part #56765] and wipe away the excess from the joint after it is tightened up. this stuff is high temperature, and works in gasoline [although i wouldn't recommend it's use in fuel systems, it works good in bulk tank applications],petroleum oils, natural gas [not over 300# pressure],butane, and propane, plus air compressor systems.

Yes you need to use some in this case. It'll also help you get it out next time.

I've heard that Loctite 567 is the very best, hands down & yes hardware store NPT fitting quality is hit or miss.

My biggest issue has been to over tighten these type situations.
Once I get it in my head it doesn't need to be quite so tight I have no more trouble.
I have never used a sealant on the rear cover drain plug.

Well, I put a little Permatex on it just to be on the safe side. The cover is an aftermarket chrome cover so it may not have the best fill plug on it. I had my Dana all gone through and assembled the summer before last at the local ring and pinion shop. It had just been sitting on the garage floor until recently. I finally got around to installing it in the car and then filled it just before my original post. Here I was worried about the fluid leaking out the fill plug only to find out once I put the new fluid in it is leaking pretty good out of the pinion seal (which was replaced when it was assembled).

I've never used sealer, never had one leak. If the fluid level is correct and the vent is open, there is no pressure in the rear end to force the lube past the threads.