I've used copper tubing from home depot to run diesel over to my 1 qt draining filter (dutton laine) on passenger side, and back to the pump, for 2 + years on 240d. No issues. I've got the copper line isolated by 2" rubber line on both ends, and clamped to firewall by tiewraps in the middle. No vegetable oil, although I do run 30% used motor oil in the summers. Have had copper from the rear gas tank to the 1 qt fuel filter on my Ford gas car for 15 years. No issues. I found metal 5/16" "brake" line would vibrate and fracture around the engine on the gas car 15 years ago. I tried to buy dead soft steel line like original transmission cooler line back for fuel use and transmission cooler line, then but nobody will sell it, they are afraid you might use it to hook up a disk brake caliper and kill yourself with it.Type L Copper line is NOT rated for pressure of disk brakes or injector lines. There is a type K copper rated at higher pressures, but not for sale in consumer quantities.

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'80 240D biege "Mercey Bird"
'80 240D light blue "Slug"
'80 300SD dark blue "Theodin"