atf can and will dissolve/soften rubber, but what most people refer to as "rubber" in an engine is almost never actually rubber. If inside an engine it is almost always a synthetic like hnbr, viton, etc.

 

Ok. Makes sense now. Otherwise it sounded like you were putting your oil in and then driving to the gas station.

 

I used the term rubber interchangeably, but yes. You're right that the seals may in fact be made of something ATF resistant. But it's just kind of confusing why people try to use ATF when there are better options:

- Diesel also dissolves carbon, you can use it to flush a crankcase and soften the oil deposits, etc. And it's cheaper than ATF.

- If you don't want to use diesel for some reason, Seafoam / MMO is less aggressive, but are specifically designed for this purpose. People call it snake oil because of the claims on the can, but it does in fact soften carbon. Just put an old plug in a cup of Seafoam and come back in a few hours to see.

- If you're just looking to build compression, motor oil is not only thicker but also actually designed to burn. So it's a better, safer choice all around.

I know there are arguments that ATF might be safe and they may have merit to them, but I don't see why anyone would use it in any situation. Other than an emergency of some sort that I've failed to account for.

Obviously OP used it by accident, so this is referring to the carbon cleaning or deflooding trick.

 

Diesel is the best solvent for carbon cleaning and just cleaning metal parts in general, its the cheapest and easiest to get. It does in fact share the same rubber softening properties though.
But as I said above rubber is very rare in engines, and has been for as long as most of us have been alive.