Don Carlton was more than a capable driver - many tales have been told of his ability to repeat -- meaning if he was asked to shift at 8200 from first to second and 8400 for the other gears, that is what happened. And he could do that all day long - a natural test driver and an excellent race driver - but to answer your question, Dick Oldfield was also fully capable of doing any testing necessary but it is always more consistent to use the same driver all the time.