That number does not ring a bell, but yes, no preload is why your front end sucked. Schedule 40 PVC
is fine. Stack it and cut it flush extended, as mentioned, and you will have 20mm of preload with the cap installed. Synthetic or not is not a huge deal. Go 10W so your damping curve is better matched to a stiffer spring. 130mm oil height in the tube with it collapsed and no spring or spacer installed at that point. Typical arrangement is a washer between the spring and spacer, and another washer on top, but with the PVC rather than the aluminum tube, the top washer is not so much of an issue.