oil viscosity in 2009 yaris

[matanfr]

hello,
I have a 2009 5dr multimode hatchback yaris.
In Israel the weather is hot in the summer and the temperatures aren't hot during the spring and winter.
when i bought my yaris,the service interval is 15,000 KM or 1 year. as stated by the dealership.
the oil viscosity the dealership put in is 10w-30 and it's semi-synthetic.

the car has made 6100 km since august 2009 (i don't drive much since i have a car from work sometimes).
i want to preserve the engine in top shape.i was recommended to put synthetic oil and i chose the ROYAL PURPLE brand (we don't have amsoil,shell,castrol in here since the companies don't import them).

has anyone here who's using RP synthetic oil and can recommend a different viscosity?the manual says 5w-30 and NOT the 10w-30.can someone recommend the proper viscosity due to the climate?

can i shortage the service intervals?