I have Atlee caps. I use auto gas almost exclusively, and have had no trouble.
My friend was in the same boat until it almost cost him his airplane. I think that the potential for this to happen is rare and takes a particular set of circumstances to create a very predictable result. Here are the circumstances.
We were both heading to the Brooks Range from Palmer so my friend and I both topped our tanks completely full for the flight. He has stock tanks (requirement #1) and filled them
full with auto gas (requirement #2). The tank that collapsed had an Atlee cap (requirement #3). He usually burns his tanks down in a balanced way, switching between tanks periodically. This trip he decided to run the tank without the Atlee cap totally dry (setting up the ensuing engine out) before switching to the tank with the Atlee cap (requirement #4????). This left the Atlee cap immersed in auto fuel (not fumes) for an extended time causing it to swell enough to close the vent in the cap. His engine out took a bit of time (30-45 minutes or so). The symptoms mimicked carb ice at first, since he could counter the slow loss of power with the application of carb heat. This was a loosing proposition since the tank was being slowly crushed by increasing head pressure. He made it across the flats to Fairbanks and the engine died on final. This was all good.
The end result was quite amazing. The tanks was wadded up under the tank cover. As I was standing up on his airstreak with this perplexed look on my face, another pilot walked by and said "Let me guess - you are running auto gas with an Atlee cap". We were able to save the trip by reflating the tank without it cracking anywhere. My friend mentioned this to Atlee himself and he was very Atlee when said his cap is not for use with auto gas.
The question marks on requirement #4 are because I'm not sure how long the gasket has to soak in auto gas to cause this situation. Collapsing the tank required a fairly unique series of events and the engine out happened because he had no other tank with fuel.
In the world of logic if a stock cub tank crushes to point of killing your engine is there any other culprit in the line up beside the gas cap and it's vent?? The incident is also not unique and I was glad that other pilot passed by when he did because he helped us with reflating the tank and getting on our way.
I did hesitate in my posting of this because of the long winded nature of this event but think it's worth mentioning. You might try soaking your cap in auto gas for an extended period of time to see what happens.
Jerry