I'm not good in technical things, but in this case I can answer ...
Go onto the right side of the Estrella (where your right foot is when you sit on the Estrella). At the front / right of the motor is an "quarter oval" plate. Behind this plate is a second plate (I never opened it, but my mechanic was astouned about the second plate)! Behind this second plate is the oil filter.
First you should drain the motor oil. After that unscrew the described outside plate (it is fixed with three screws. The inner plate is fixed with 2 screws, you should use a rag, when you remove it. The oil filter itself is a paperfilter. You should get it in every motorbike accessory shop. Changing oil without changing filter takes 1,8 l, with changing filter 2 l of oil.
Thank you Soulie. What filter do you recommend? K&N? As for oil, I assume 10W40. Both Castrol and Motul are available here. Many of the big bike riders use Motul.
Every son would have his motorcycle, And daughters ride away on casual pillions.” - T.S. Eliot