An ultrasonic bath is highly effective removing caked on lubricant and grit. I use a capful of cleaning solution in water, or a similar amount of Simple Green cleaner. Neither contain EDTA, which is used to remove stains and oxidation from brass.

You must dry the assembly thoroughly (compressed air is best) and relubricate it. The bath cleaner will strip all oil and grease from the mechanism. CLC lubricant and white TFE grease (W25) applied with a swab do the job for me.

Ordinarily I would recommend complete disassembly of any parts subject to ultrasonic cleaning. However a drop-in trigger is sufficiently accessible once removed from the lower.

In terms of replacing the trigger in an AR, my preference is to a three (or more) piece assembly using the full-length pins through the frame as the OEM. The advantage of a pre-assembled trigger is best reserved for precision and reliability in a bolt-action rifle.

I installed a Geissele standard-pull trigger in my Colt M4 (16.5"). I couldn't be more pleased with the results. It has a two-stage action with a net 4.5# pull, but the second stage is only about 1.5# more than the take-up. In effect, it is a safe tactical trigger with a target trigger feel.