How often to change oil?

[wawa]

Hey all

Newby question. I've look in the owner manual about the oil change frequency and all it say is: Oil change should be done frequently as per your schedule maintenance.

But I dont have the schedule maintenance, so how often should it be done. The last 2 car I bought were schedule to every 8000km (5000miles) Honda fit and Scion tC. But my older MR2 MKI was at every 5000km (3000 miles) So with a Posrche in between in age, should I do it at 3000miles or 5000miles?

Thanks
Chris