"Zephyr" writes:

It is highly recommended that "if" you use the older copper lines with the
newer gas, you should be aware that the manufacturer recommends less than 5%
of mineral oil be allowed to mix with the newer gas. This concentration can
be tested with a refractometer. It is better to run new copper, but if you
have to use the older lines...... 5% is the highest concentration of mix
allowed.

In cases where it's difficult to run new lines, can the old ones be
flushed with solvent to remove the old oil?

Dave