Never tie knots in dyneema or spectra. it will cut strength in hall. only q power line can be knotted. btw a figure 8 in nylon rope gives you about 90 percent strength of line but this exact same knot in spectra will snap the line. so it's not the type if knot but that spectra or dynema can't be knotted without snapping
Spectra and Dyneema are brand names....not different types of cordage. They are both made from UHMWPE fibers. And a figure 8 knot exerts the lowest amount of pressure on the line but you are right that the line will eventually fail if the knot stays intact for a couple of years. Like I said the best method is to get newer pre-stretched lines and just replace the old lines he is flying.