Which elements to forecast/estimate Oil/Pretroleum derivatives, would you consider?

I mean, if you had to forecast the price or production volumes of polyethylene or polyurethane, for example, which elements would you include to the model, in addition to oil crude prices/volumes?

The idea was to biuld an econometric model, but I have doubts of which factors I could add into the model, since the focus is to predict oil derivatives and not crude oil per se.