I just installed all new brake lines, BUT did not replace the old master cylinder which was drained nor the wheel cylinders. This is on a '68 malibu with 4-wheel drum brake system. The question is, can I use synthetic fluid or should I stick with the regular type? Would like to do this in the next day or two.

You can switch to synthetic DOT 5, you just have to bleed it a lot to make sure all the old stuff is out. That's what I did years ago and haven't had any problems.

Thank you. I was afraid that there would be enough of the old to cause me problems.