Done III, Q3, which was actually quite nice, but felt very "old style Cambridge Entrance Exams".
In fact, I feel looking at these papers in general that 1996 and earlier marks a bit of a "watershed" in terms of content. The questions require more and more algebra grinding, it feels you're supposed to know a lot of stuff outside the syllabus, and so on.
Going back previous to 1996 it seems STEP I, STEP II also take a bit of a quantum leap - I looked at the 1990 STEP I paper and wondered if it was a mislabelled Paper III (questions on moments of inertia?!?).
So my feeling is we're hitting a bit of diminshing returns if you're looking at practice for the current style of STEP exams. It's probably more useful to complete all the later papers (including mechanics / stats).
Any thoughts? (As I'm just doing this for fun, not exam practice, it doesn't hugely matter to me, though the "fun level" of the 1990 questions doesn't seem very high)!