There is no need to look down while riding, it depends too much on the RPM. The real time to check the oil pressure is in idle, like before you turn it off.
Still quick glance down is faster than looking over your shoulder to verify what you see or not see in the mirror and every biker does that.
Just saying 
I've been guilty of constantly verifying what I saw in the mirror prior to returning to the lane after a legal passing of a car. Never know what the other one is thinking. I've had courious drivers speed up to look 👀 you over. Most riders I know don't actually read the gauge once underway. They just glance for a familiar needle position reference.
