Oil additives - snake oil?

So do the STP or Marvel's Mystery oil or others really benefit anything?
Like loosening rings for example.

STP helps if you have an engine uses oil. MMO helps clean up carbon and lubes the upper part of an engine and can be put in both the oil and GAS but ATF is cheaper and does the same thing

I second 'old 'comment! To loosen piston rings; Marvel Mystery Oil.. Use in crankcase oil and in the diesel or gas; It-works..

Do you have reason to believe the rings are stuck? Has it been stored with water in the cylinders? Extremely carboned from neglected oil changes or lots of idling?

If so, you can try, but if the rings are truly stuck, the longer it's run the more cylinder damage it will cause.

Stp has zinc in it .
I called them , they said they dont recommend it for tractors cause they never tested it on tractors. ?You decide !

I use mystery oil n my oil can to lube dry pistons from fuel flooding ,or engines that have run in years .

To all who replied, Thanks.
I'm going to use some Marvel's in the oil.
This is in my 38 Twin City KTA. It runs well but does puff blue smoke at times. At idle mainly. I can live with that but thought I'd try an additive in case some rings are tight.
It has been sitting for years w/o running.

I'm sure others have seen this (or something similar): At all the big farm events around here there's a Lucas rep who sets up a booth and has a big 'ol Cat diesel on a stand running away at a low idle. He claims they run it in the morning for a few minutes with Lucas heavy-duty oil stabilizer in the crankcase (he admits they use 100% stabilizer, rather than part oil and part stabilizer like you're meant to). Then they shut it down, drain the oil, leave the drain plug out, and it runs away all day with an empty crankcase.

When I saw it at the IPM a few years ago a few of us where rather skeptical of it. I assumed there must be a false bottom (or similar) in the oil pan. For our benefit the rep shut it down, took out an impact driver, popped off the oil pan, and fired it right back up. Sure enough, there wasn't a drop of oil in the pan when took it off, and we watched it plug away happily with all the lower end exposed. When I walked by on my way out a few hours later the pan was still off and it was still plugging away happily.

It sold me. Whether or not there's any real benefit to running it in tractors vs. straight oil I'm not sure, but he won my support - even if just for the showmanship.

I will say this for it as well: Lucas recommends the same stuff used in transmissions to smooth shifting. On any old tractor that takes gear oil in the tranny, I put it a good dose of it (at least 20 percent) with the gear oil, and it sure does smooth out the shifting and quiet them down a lot.