How do you add oil to polaris sportsman 500 engine oil?

This depends on what model year the sportsman is.

for older models, the dipstick/fill cap is a yellow colored cap located under the left front fender. It is tricky to refill. your best bet is to get a cheap funnel and clamp a piece of plastic hose to it. Snake the hose through the hole in the side panel of the ATV into the oil tank to fill.

Newer models have a black fill cap / dipstick located near your left foot as you sit on the ATV. This new design is much easier to get at and requires no special funnels.