Because the oil in the bottle is that weight. It is 5 weight at startup and then becomes 30 weight at full operating temperature. The W stands for Winter.
5w30
Your Holden/Opel Astra requires an ACEA A5 oil, check the bottle for that spec in a 5w30 weight oil. Synthetic oil can either be 0w30 or 5w-30.
5w30
5w30
5w30
5w30
5W30
No, not if the recommended weight is 5w30. Only use the weight recommended by the manufacture.
5W30.
5w30
5W305W30
5w30