ATF doesn't hurt seals, it's oil, very light weight oil. Has additional additives and cleaners but oil none the less.

Probably not, but I had ATF swell polyurethane badly due to the additives so am wary...

Now that is a use I will follow. I have always used "air tool oil" for that. But when my little bottle (with drop stem) is empty, I will refill it with MMO.
When I was running Alky injection, I stored the pump during winter by filling it with MMO and putting the pump in a zip lock bag.
It helped keep the pump seal from shrinking (drying out).

Engine oil will destroy rubber motor mounts and suspension bushings as well. I would imagine it would do the same to polyurethane.

I recall an episode of Chasing Classic Cars and Wayne's old-time mechanic, Roger used MM to unfreeze a locked up engine in a car they had acquired. Took him several days using MM down the cylinders and gently rocking the car while in gear, to eventually free the motor. Apparently that's been his go-to method for some time. Regardless if you like the show or not, Roger's da-man and deserves major creds, especially considering he's in his 80's.

I had a collapsed lifter in a 350 Buick back in 1980. Threw a quart of Marvel in and in a few hours all was well!!
Ron

Yep, I had a Aspen with rackety lifters and off the advice of a friend, poured a qrt of trans fluid in. 2 mins run time and all was well. Same results w marvel

CD2 Lifter Quiet was the best, but now unobtainium.

Have not seen C D in a LONG TIME

I have used it in engines that have sat for years. It is good stuff. For fuel systems and engines lubricating.

I'd like to try it on a loud lifter car at work. Do you add a quart to an engine, generally speaking? Do you add in place of one quart of oil at oil change, or change oil and refill with same amount of oil called for, plus MMO?

Just throw the quart in on top of the normal level. Done it plenty of times, nothing to get scared about

Just don't leave it in there. Change the oil and filter after a bit.

Ohhhh. Bummer, I can't do that.

If the engine dirty, yes

I normally run the 413 with 6qts 20-50 and 1 MMO, as per RIck Walker

I'm sure it's dirty. 5.7 with over 100,000 miles and I haven't checked idle or engine hours, but for sure very high. Not the best maintenance here. It just came out of fleet for lower rad hose and I think they did oil change while there.

AH HA! Ok, I was waiting for one of these to rear its ugly head-- this will not work on an MDS engine unfortunately, 99% of the time thats just a down right failed lifter usually taking a camshaft with it.

And not to start any kind of tinkle match, but I've ram trans fluid and MMO in oil for the duration of the life of said oil until its next change.

I happen to know this poor car got ran without oil for a period. Damaged oil pan, gasket blew out....long story but it got driven back to shop with oil dumping and had to be out of oil or close to it by time it got back to shop. Nuff said on that. No it was NOT me. After this is when tap tap tap started. And remember, over 100k hard miles and tons of idle and engine hours.

Pilots and aviation mechanics (A&P) use it frequently for it's ability to dislodge glaze from cylinders and heads. Remember aircraft are air cooled and meticulously maintained (should be at least).
The stuff is basically a lot of mineral oil and some mineral spirits with a little bit of benzene solvents to cut oil that becomes plasticized from high heat (think outside cast iron skille)t.
Anyplace you see that crusty black stuff or amber colored buildup n parts when disassembling an engine will be cleaner if you use this stuff in oil and fuel. Using it one needs to stay up on oil change intervals and filter changes. If used on old engines checking the oil pickup screen is a good idea. I recall seeing "experienced" engines that died of oil starvation after the stuff was used and the pickup was plugged solid. Regardless wearing surfaces generally stay clean (cam lobes and lifters, cyl walls, bearings) and the area around ring lands do stay clean.
If using synthetic the best I method I recall was to put it in about 1,000 mi before a filter change, repeat every change.

My gut feeling is the benzene solvents really do the work and the mineral oil and spirits thin and mix with the oil to get the solvents into the lube circuits and carry the crud away to the oil filter.
Which reminds me use high quality filters so as to trap the crud.