I've just had a pressure relief valve changed on a Sime combi system. My engineer did a partial draindown and refill, but when I asked about topping up with inhibitor, he specifically warned against using it on an older system (my boiler is 8 years old but the rads are probably 20 years plus old, the system was flushed when the boiler was replaced). He said that on such an old system it was only the sludge that probably stopped the rads from leaking, remove it and you are likely to develop leaks. Only use if rads have developed specific cold spots (which they haven't). Is this an old wives tale, or good advice?