Opel says every 9375miles (15 000km) with 10w40 semi synthetic oil
or once every 18750 miles (30 000km) with 5w40 full synthetic oil
and I do alot of miles too with her at the moment, I never really had to top her up except when I was due for a service so in my experience its pretty accurate !
btw I mostly do city traffic so...
I think that if u make sure ur water AND oil are warm before revving high and just in generally treat ur car with love (so not trashing it every day or try to reach top speed on the highway, etc) that ur safe !
remember the z22se was designed to keep service at a minimum 😃
changing oil sooner wont harm her of course but if u do it too much ur just throwing away money and oil
just my 2 cents ^^