I'm convinced that you can have fairly different sight glass readings with the same amount of oil in the engine. Drain and measure is the only way to be 100% sure what you've got. My proof of this theory is that mine used to routinely "make" oil- low at one check, higher at the next, none added in the meantime.
I also think that the valve guide seal issue can come and go. They go up and down with the valve for a while- crazy consumption. They reseat on the head for a while- no consumption.