Loss of CO2 through tubing at low pressures is negligible. People say use this tubing or that tubing but it really doesn't matter. The reason to use one tubing over the other is longevity. For example, some tubing hardens and degrades over time due to exposure to co2. Vinyl, for example, will really start to stiffen up. Permeation of co2 at 5, 10, 20 psig isn't an issue. I got some Tygon for cheap, so that's what I use, but before you go out and spend $50/ft on specialized tubing, think about whether it's worth it to spend $25 on tubing only to save $0.25 in gas.