I have these http://www.mcmaster.com/#5792k32/=13s9wwj Polyurethane clear tubing for return lines on my 85 for about 3 years now with no leaks. They don't stay clear very long and turns to a light to dark brown after a year or so.

I also had the same lines on a WVO car and the WVO makes the polyurethane harden over time. I had one leak and snap off at the barb.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked