How many miles on the engine since the last rebuild, or if original, how many miles? Royal Purple is synthetic, is it not? Try returning to regular refined oil of the viscosity specified for the temperature range where you live. That may reduce consumption.

In a previous century, my wife's 1968 144 aged to the stage where it used a lot of oil. But we lived in Houston,and you know the summer there. So we used 50 weight oil and that eliminated the consumption. This worked for two years until we sold the car.