The plot thickens...
I have looked again at the oil that I drained from the tank. Whilst it appeared purple when it drained, now it has settled, I can see that the oil is a relatively normal colour, but there is a heavy purple sludge that has settled underneath the oil. Furthermore, sticking my finger inside the filler cap, and feeling the inner wall of the tank, there is a heavy, very sticky purple residue all over the inside walls of the tank.
I am even more confused than before....
Any ideas?
PeteR