I bought a 20A bianchi magazine pouch. The magazine
fits very tight, so I sprayed the pouch inside with silicon, which helped a little, however it tightens up again on the magazine. I tried some Mink oil, and it's still too tight.
What is the best thing to use ? I didn't want to stretch it
out(another option) and have the magazine falling out, as
this model has no snap or flap.

First, take your magazine and insert it in and out several, several times so that you can establish "etching" on the inside of the pouch.

Then, wrap your magazine up in a ziploc bag, insert it into the pouch and let it sit for about a week or so. It should stretch it out just right.

Thanks I'll try it.

I use an old sock over gun/mag. Just adds a little and works well IMO