Possibly a stupid question.

I've always being using Castol long-life 5w-30

But now the dealership has stopped selling it, told me they only stock VW 0w-30 504/507   (instead of Castrol branding, its a stock grey VW bottle)
Product code: gs55-545-m4

Is it ok to use?

Yes.

504/507 are the key indicators to look for.

If you'd still prefer a 5W30, I'd suggest the Penrite's Enviro+ , a VAG licenced product from Supercheap or LiquiMoly 5W30 Top Tec 4200 from Harry's Euro.