I started driving back in early 70's and am hard time catching up with the latest and greatest technologies. My wife just bought 2014 Prius v three and the manual calls for 0W-20 oil!! We are in California and not in Alaska or North pole. 0W-20 is too thin to protect the engine when ambient temperature is hot, in fact, yesterday it was 93 degree. I have a service manual for 1973 Nissan Sentra and there is a oil viscosity chart (I wish I could post it here) that says even 5W-30 is for -30 to 60 degree range. For 0 to 100 degree range (typical California weather is 32 to 100), 10W-40 is recommended. In fact, I have been using 10W-40 on all of my cars so far and have never had any issues associated with oil. What has changed so that now 0W-20 can sufficiently protect the engine at 90 degree F?