The IST aluminium bp is fine, I use one in summer. For winter I use Kent tooling 5mm SS to counter the extra under clothing. I've been using DS4's for about 5 years now, no issues what so ever, that's for tropical sunshine dives and freezing UK quarries in winter. I've not experienced a free flow in them yet. Oh and I use a midnight as well, although I've moved to a Halcyon explorer for two or more stages.