depends on your engine. check your user's manual for the optimal oil weight to be used in your car.

for example, the the rav4 1.8L 1zz-fe engine's optimal oil is a 5w-30. while the rav4 2.0L 1az-fe engine's optimal oil is 10w-40.

to determine your the optimal oil, check which oil has the broadest / widest coverage of temperatures. though you can safely rule off temperatures of below 0 deg C here in our country.