That is a good thread about liners. I hadn't read it before, but the information in there was the reason I said "the thicker the better" instead of referencing a specific number.

When we bought our pool Doughboy was either manufacturing their own liners or having them manufactured to spec. I do not know if this is still the case but it is worth looking into before anyone buys Doughboy. Our liner is 25ml and it is holding up very well in our extreme climate (100 ish some days in the summer and -40 some days in winter) after six years. I noticed some mild bleaching below water line this year which doesn't disturb me at all since I keep my chlorine close to the top of the scale. In fact, it is possible the liner isn't bleaching below the water line, but may be darkening above the water line due to weather exposure and staining. I cleaned it this year, but it may retain some stain.

If this helps the OP... we bought our pool as a package. It was not extra for anything other than install. Everything was included right down to skimmer, vacuum head, filter sand and some start up chemicals and a solar blanket and winter cover. The 25ml liner was just a standard part of the package.