I'm gonna be replacing the 60-yr-old 3/8" copper tubing that runs
through our concrete basement floor from the oil tank to the burner.
I'm gonna chisel out the old stuff, clean-up the channel, and then
re-bury the new tubing with concrete.

Anyway, from what I understand it's advisable to first encase it in
some type of outer plastic tubing or conduit to prevent corrosion
(that's why I'm replacing the stuff that's there). Any recommendations?
I would think that 1/2" Sched 80 PVC would be OK, so long as the temp
never gets above 140 deg. near the burner.