Kel-Tec magazines are made by Mec-Gar and these folks produce some of the best magazines made.
As stated, there is no problem leaving your magazines loaded as long as they are kept clean and not allowed to load up with dust, lint, or rust.
Also as stated, spring cycle, and not compression, causes fatigue.
Oiling the magazines will make the problems worse and usually ends up killing the primers of the cartridges loaded in them.
If you have a major concern of overstressing the spring by keeping it loaded, just download the magazine by one round.