Tires always lose air in storage is this normal?

The super expensive Continental 5ps on the C63 lose air at about 10 lb ovr three months indoors.
There use to be some compound you can spread on the inside of tires to stop leakage. Western Auto sold tires that had a red liner in it for that purpose. they're gone of course.
Jut get a good gauge. I bought all the best gauges and not one of them works well. The expensive Pirelli brand dial indicator is off by as much as four pounds too low.