First of all never listen to people who ask you to shift oil to a thicker viscosity if your current oil is not giving any problems. I have seen sales people sell 20w50 oil to only 2-3 year old vehicle owners. My civic is almost 3lac km and I am still using 10w40 (Zic A).
Additionally sometimes they ask you to switch to other brands because they get more commision/profit margin from them.
About Zic A+ I agree may be a little thin (5w20?) I think?
They have renamed the branding from A, A+ etc to X7 (for both).
You should check for any leaks. Maybe filter or the drain bolt is leaking.
What do you mean by "no engine oil is lasting more than 1500 km" How many times have you replaced the oil since then?