Importance?..
In days past, that was really important due to the different additives that were used between makers. But, I've recently read in the world of synthetic oils, it's not exactly super important or necessary.
If you like to change to a full synthetic oil, you might just drain the oil tank, then refil with the synthetics.
The oil I use, "Quicksilver full synthetic" actually states on the back, it's designed to mix with other oils. But, there's a clause with words to the affect that you can mix going up to a higher brand, like a synthetic but not downward into a mineral.
Chester and Dr. Honda both have a lot of experience in this area, maybe they'll have differeing opinons that may give you more options. I guess, if the question is there, you could always change it out.
I used an old shop vac to hook up to my rotary chamber lines. If you drain your rotary and tank, I don't see a reason to drain your oil injection lines. They are primed and will use what oil is there and will have an automatic changeover when that oil reaches the cylinders........:cheers: