If use fully synthetic lub you can anytime use it up to 10k km (include idle time engine running). Imagine Merc and BMW service interval is 12k km and VW is 15k km, for normal car using fully synthetic definitely can use for 10k km. Yes, the Shell Helix HX8 is good even for up to 10k km interval.

btw to share some info about lubricant Fully synthetic does not always equal to 100% synthetic oil. In market MOST fully synthetic oil is not 100%. Those lub written is 100% fully synthetic then it is trully 100% synthetic. Many famous brand is up to 90% which is still very high synthetic content while some unknown brand although they are fully synthetic but the content can be only 50%. For semi-synthetic or synthetic oil the content is about 30% usually.

To buy engine lubricant recommended to go workshop or tyre shop to buy. If buy online then choose the official store. Try avoid buying from sparepart shop because genuine oil they sell to workshop or tyre shop while counterfeit one will usually sell to walk in customer.

Among the engine lub and oil filter, the engine lub play more important role. Fully synthetic from premium brand can anytime use more than 10k km even for Japanese car. I worked in automotive industry for over a decade and can assure not much issue on the oil filter that will cause need of change the engine lub before 10k km for fully synthetic. Even if we using the counterfeit engine lub, it will take at least 20k km or 30k km (2 or 3 times change of counterfeit engine lub) only it start giving bad symptom like louder engine noise, less smooth of driving etc. By 50k km onward the damage will get worsening and more apparent.