I’ll try my belt
The yoyo construction is based of three separate item, the body, the bearing and the string
In solid axle time, the important is to have a good string tension, too tight, can’t sleep well, too loose can return
The design of bearing is to reduce friction between the body and the string since the direction of the play is mostly long spin trick
So to spin long, the string has to be independent to the body. The string still have higher friction to the bearing outer race, whole the inner race is screw tight to the body
The thing separate both races is the miniscule balls 7-10 depending on brand/model. So the tolerance of there balls between the two races is so tight thats a file of oil is actually impeding the ball from rolling in the race groves.
The lowest friction here is a flat surface against the spherical balls, and usually the groves is in V shaped to hold the balls inflate. Due to the nature of oil sticking to both surface, this making the inner race pulling on the balls, which in turn pull the outer race along with the turn. The string being “stuck” to the bearing this follow too turning into the gap. Enough string will catch on to the response pad and bring the yoyo back up to the hand.
The lower the viscosity, the thinner the oil fill will coat the bearing balls at well at the inner groves. Best unresponsive play is to play dry, but being direct contact, the inner surface at well at the balls does get worn down overall time. Lube help extend by reducing less direct touch between the metal.