When I was testing patterns for this, I ordered some of each type of vinyl my local store had available: 8, 12, and 20 (my local place was out of the 4 gauge). After sewing with and laundering them, I really feel strongly that if you want your mask to stand up for multiple uses (which is important in this teacher household), you need to use at least a 12 gauge. My preference is 20. Most tutorials I see recommend 8. The 8 gauge vinyl is cheaper and sews up just fine. Where you see the difference is in repeated wearings and washings (this style mask, and all masks, need to be sanitized and washed regularly). From hand or machine washing, the 8 gauge begins to get ripply/wavy across the surface. Here is the comparison after 1 washing (20 gauge in the blue mask on top, 8 gauge in the purple mask on bottom):
If you do wash in the washing machine, the 20 may develop a big crease/wrinkle if it gets folded over. You can remedy this by heating the mask up briefly (in a dryer, in an oven at 200 F, or with a hair dryer. NEVER in a microwave) until the vinyl softens and then lay it flat until it hardens again.
You can do this with the 8 gauge as well, but I find you need to do this repeatedly and over time, the vinyl distorts and is no longer a flat sheet.
All vinyl masks need to line dry. They should not be put in the dryer for a full cycle.
Let me know if you have any experience/observations that help others as well! Thank you for making these very important masks!