Fuel filter has to be changed after like 30K km, not with every oil change (pic of filter attached for reference, as it is often neglected; around 500 rs). As for your oil, if you do not see any leaks or there is no reduction in oil, switch to 10w40. Also put half a bottle of LiquiMoly MOS2 (around 1 thousand rupees, lasts for two oil changes) along with your oil when you change your oil next time. It will make a protective film on your engine's internals, which will reduce friction and also protect your engine, engine's sound will be reduced and you will feel it running smoother. Then add the other half of the bottle in the 2nd oil change. As for your friends, a 20 year old engine, driven around 280k giving 11kmpl is not bad at all, my efi 2002 exis city gives around 12 as well.
Fuel Filter

LiquiMoly MOS2