oil use to be by product of crude oil refinery and had a lot of sulfur and other nasty stuff in it, today's oil are engineered and much more pure or synthetic.
if you engine is operating correctly and oil level is fine 10 to 15k per oil change should be fine. check for oil color and smell change.
viscosity measurement is easy if you want do it , use a measured cup ( lets say 100 ml ) and drill a hole at the bottom , fill the cup with oil and measure how long will it take to empty ( at a given temperature and pressure ( ambient ) ) . does it change with time ? it mean oil is breaking down.
above measurement device is called a 'ford cup' , you may be able to buy it online.
P.
