Although it could be described as normal once upon a time, it is a sign that the oil has thinned too much - back in the day there weren't other options with oil choices and the recommended grade was more prone to degrading / becoming too thin when hot or old. Obviously the risk here is a lack of proper lubrication at any revs, as it has more of a tendency to 'run off' components rather than coat them.
It may be worthwhile moving to a more modern equivalent that should withstand the heat better, and see if you get a similar scenario. Alternatively it could be that your engine isn't cooling effectively, or even something as simple as your idle speed is set too low.