If your recirculation line has no shut off valve after the needle valve you do not need high pressure tubing there. The inlet is HP but the other side a rather low pressure, as it feeds the HP pump. The exception might be if you have a HP recirc. pump, but that is unlikely, especially on a home built basic RO.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.