ok, then its nothing to worry about.. just keep on changing oil between 2500km to 3000km mark and engine will deliver best
"W" represent the viscosity measure of oil. Viscosity in simple words is the thickness of a fluid or say resistance of a fluid... read following link for details
Viscosity - Wikipedia, the free encyclopedia
so 20W and 10W represents the thickness of a oil... means 20W is more thick as compared 10W
whereas the other part (30 & 50) represents the temperature/heat resistance of oil... 50 is more resistant to heat as compared to 30 or 40
Its better to use 20W 50 over 10W30 specially in summers as 20W has more viscosity and more heat resistant!
Thicker oil has a slight disadvantage of sustaining oil-pressures and faster draining in engine block... but its not much to worry