Long time no post - I'm back on the GTI wagon with a nice '89 GTI 16v.
The car has been well looked after but one thing that bothers me slightly is the fact that it's got a black rather than a blue expansion bottle cap. The bottle on this car is the older square type and the new cap is the round one pictured below.

Not had any problems to speak of with it, but I've got to order a few parts from VW Heritage and wondered if it's worth swapping it out with one of the blue ones? I seem to recall that the blue ones had a better pressure relief valve that was less likely to fail in them.
Thoughts welcome!