I think go for Shell HX-7 (10W-40). it's blue bottle, there is also shell 10W40 with HX-5 (Yellow). go for HX-7. One fellow is trying this, shifting from GTX, shall ask him difference. I saw people liking ZIC a lot and ZIC A is 10W40 with latest API SM. There are two standards in parallel one is SAE (mainly the multi-grades mentioned as xxW-yy) while the other is API, SL and SM are current standards.
But shifting to 10W-40 is not dangerous, if your rings, piston are right. It's a bit thinner at cold than 20W, so pumps easily and allow easy starting. after heat-up, it attains viscosity level of SAE 40 approx.
Check the prices of both Shell (Metro ISB: Rs. 1,450 / 4 Ltrs.) and ZIC and post as well. For the driving period you mention, 10W40 is even better, or 5W40 also. bcoz more viscous oil, with this much idling, would leave more wax/sludge to engine and less viscous would lesser.
Actually dear, I have not tried many products yet, myself experimenting and searching and studying materials from different places. Many friends who have car don't even know about grades they are using... they know caltex, shell laal vala etc. etc.
See some excel sheets in file
2shared - download Oil Comparison.xls