i20 BJ 2010 Question about oil change, spark plugs, rib belt

Hello together. Also I would like to wait for my i20 BJ 2010 (140000km, 78hp, petrol) in future as much as possible. For oil change: Which oil do you recommend me at 140 000km? So far the recommended maintenance specification 5/40 has always been filled. Should a thicker 10W 40 be filled in after so many KM in the future? And is the interval 12months still current? Drive approx. 15000km per year. For spark plug change: With our Opel Corsa D this is quite easy. The i20 is a single one (not like in the Opel in the block). Is there possibly a video/guide? V-belt: This was last changed according to maintenance book at approx. 75 000km 4th year (flat belt). Now it would be due at 8 years. Flat belt looks still “good”. No visible cracks / grooves etc. Is the effort in the workshop large b The tensioning of the belt should be the single problem… Is there a video/instruction here? Exhaust system is not a problem. Thank you very much!