I was told by the toyota service department that the break-in oil was 5w-20 dino oil. I still switched to 0w-20 full synthetic at my first oil change to protect the valve train during cold startups. I got slightly over 1 mpg increase in mpg after the change.

I ask the toyota certified tech about running 0w-20 year round. He said thats not a problem. The 0 works when the engine is cold. Once warmed up it's running on the higher number of the oil.

The temperature here in the New Mexico desert Summers gets up to 105 degrees. The man said it should just run fine, even in the heat.

Synthetic generally has a much higher film strength than conventional oil.

I read from a Amsoil site that their 0w-20 will pour down to -65 degrees and will protect the engine up to 475 degrees.