a 5w30 is synthetic and far more expensive with no gain except 1% of fuel consumption, which is nothing... No need to invest in synthetic oil to change it every 5000 km...
Greece is not Alaska, a 10w40 semi synthetic will be fine enough...Better than a 30 in summer, and ok in warm winters.
If you want to be sure, run 5000 km with a 10w40, and send a sample to Blackstone for UOA (it cost 30€ with postage) and look.
5l of 10w40 + UOA will be less expensive than 5l of synthetic 5w. And it is better to have an UOA than to do a blind choice...
Oil prices and common grades are far different in Europe than in USA, and what it is cheap in USA is not in Europe... 10w40 is the best compromise in europe between quality and price, and good enough for 10 000 km OCI. Synthetic oils are far more expensive en Europe.